Allow sqlite3_snapshot_open() to be called to change the snapshot after a read

transaction is already open on database.

FossilOrigin-Name: 41399169954f9bef53c3fa89879f39823b80bd127f76cf60abbe24217878a571

diff --cc manifest
index 63a45db0462f676db6749fc79b4f4987e8befff2,ab55cf5445b1e7008cc9ca0d750a33244275a42c..1850b53a41f4a95663e1fc71190f15851a3f7d6d
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Fix\sUPSERT\sso\sthat\sit\schecks\sthe\starget-constraint\sfirst\sand\sfires\sthe\nDO\sUPDATE\sif\sthat\sconstraint\sis\sviolated\sregardless\sof\swhether\sor\snot\nother\sconstraints\sare\sin\sviolation.\s\sThis\saligns\sSQLite\sbehavior\swith\nwhat\sPostgreSQL\sdoes.\sFix\sfor\sticket\s[908f001483982c43cdb476dfb590a1a].
- D 2018-08-14T15:12:52.710
 -C Allow\ssqlite3_snapshot_open()\sto\sbe\scalled\sto\schange\sthe\ssnapshot\safter\sa\nread\stransaction\sis\salready\sopen\son\sdatabase.
 -D 2018-08-06T17:12:36.835
++C Allow\ssqlite3_snapshot_open()\sto\sbe\scalled\sto\schange\sthe\ssnapshot\safter\sa\sread\ntransaction\sis\salready\sopen\son\sdatabase.
++D 2018-08-15T14:03:26.955
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 0a3a6c81e6fcb969ff9106e882f0a08547014ba463cb6beca4c4efaecc924ee6
@@@ -459,10 -459,10 +459,10 @@@ F src/hash.c a12580e143f10301ed5166ea49
  F src/hash.h ab34c5c54a9e9de2e790b24349ba5aab3dbb4fd4
  F src/hwtime.h 747c1bbe9df21a92e9c50f3bbec1de841dc5e5da
  F src/in-operator.md 10cd8f4bcd225a32518407c2fb2484089112fd71
 -F src/insert.c 894594952bcda1dc6e1549871e4022517563545ffc7a3f4e9e5f3faa788893fd
 +F src/insert.c c723716f0de7aa0a679300f7d3541c89645f4a9882161cecdb3093fc07f8cc4b
  F src/legacy.c 134ab3e3fae00a0f67a5187981d6935b24b337bcf0f4b3e5c9fa5763da95bf4e
  F src/loadext.c 6aae5739198d96c51ae6eb97c4a5b1744c22ed7a5a565a5399a717780d48a36b
- F src/main.c df233667bbb6f05a8492ea93e0995abbeb816eab53e51e638a0dece1de0e83a3
 -F src/main.c 3c7f2159687af1b1b0d0e0776f888adb94534e0bc52e7a1a1509f83e24cb6297
++F src/main.c 187011ff7a091ff4b8bea9481a42789c0fa094715b4e5d89352fb63377673490
  F src/malloc.c 07295435093ce354c6d9063ac05a2eeae28bd251d2e63c48b3d67c12c76f7e18
  F src/mem0.c 6a55ebe57c46ca1a7d98da93aaa07f99f1059645
  F src/mem1.c c12a42539b1ba105e3707d0e628ad70e611040d8f5e38cf942cee30c867083de
@@@ -482,11 -482,11 +482,11 @@@ F src/os.c 8aeb0b0f40f8f5b0da03fe497066
  F src/os.h 48388821692e87da174ea198bf96b1b2d9d83be5dfc908f673ee21fafbe0d432
  F src/os_common.h b2f4707a603e36811d9b1a13278bffd757857b85
  F src/os_setup.h 0dbaea40a7d36bf311613d31342e0b99e2536586
 -F src/os_unix.c d9cf5ae0c79f31019d8325e8736c83914aeed64d8327a8d91a62b6439b748948
 +F src/os_unix.c e681b2a3ab1085be3eb2e81254449782ca0bd0c38b73c48cb0c2480b8f2f25b9
  F src/os_win.c 070cdbb400097c6cda54aa005356095afdc2f3ee691d17192c54724ef146a971
  F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
- F src/pager.c 76d29b8a960dcb8b67210f095899d91e4a90673a6674ea58cfd1115b705a7fb9
- F src/pager.h c571b064df842ec8f2e90855dead9acf4cbe0d1b2c05afe0ef0d0145f7fd0388
+ F src/pager.c 705de01dff9c3df9739c37a6d3b58cd2b1734fdabcef829b16cdc7721a9eeaa4
+ F src/pager.h ecc554a55bc55d1c4ba5e17137b72e238e00bd81e72ff2662d8b9c8c10ae3963
  F src/parse.y 704c94624d41d7d46a5467574130e55aa8029a563f4df538f0121475eae46e34
  F src/pcache.c 135ef0bc6fb2e3b7178d49ab5c9176254c8a691832c1bceb1156b2fbdd0869bd
  F src/pcache.h 072f94d29281cffd99e46c1539849f248c4b56ae7684c1f36626797fee375170
@@@ -580,8 -580,8 +580,8 @@@ F src/vdbesort.c 731a09e5cb9e96b70c394c
  F src/vdbetrace.c 79d6dbbc479267b255a7de8080eee6e729928a0ef93ed9b0bfa5618875b48392
  F src/vtab.c 678992ac8ec677a3f9b08126aaf891441083805e3b42574e3654d44538381c14
  F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
- F src/wal.c c617d78715e85d81fe5719b276d32186a37eb47a49d07a3d55ddbc541de043c9
- F src/wal.h 8de5d2d3de0956d6f6cb48c83a4012d5f227b8fe940f3a349a4b7e85ebcb492a
 -F src/wal.c d0d541116c378937fad99d89737cf36cfc67ff94ba1d29b2bf93bc7333e07e25
++F src/wal.c e4bcbd90072e9626126d6f3b8411159a0b984c1b9628d15237776578d5eda12d
+ F src/wal.h 606292549f5a7be50b6227bd685fa76e3a4affad71bb8ac5ce4cb5c79f6a176a
  F src/walker.c ba7225773931760cf60bf22f34d0cce2588df7ce5ce0f215a52eb88234b55ac4
  F src/where.c 155809967fbab889374dedf970ea6561b8fb519fcb165d6ba00776552ecc5cde
  F src/whereInt.h b90ef9b9707ef750eab2a7a080c48fb4900315033274689def32d0cf5a81ebe4
@@@ -1754,7 -1755,10 +1755,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P e3ea43dabf099dc2954c23d348638e7b2a8b9122d2994154bc649a2c09260c46
- R 57b28032d1f7240227e60efc94d5cf5d
- U drh
- Z f4b7118e2c3ed77a16d581f95e6f4881
 -P 20c995d3f0f4de5410962172cb59da0f25edf0c62e199420186cc59ea874e981
 -R 462315bcd4a2eb76f7dc221ad2f185d1
 -T *branch * exp-snapshot-open
 -T *sym-exp-snapshot-open *
 -T -sym-trunk *
++P 529fb55e3d00472f13446117527b0896827b11e870b581af7fe7cbb7392ef3cd 051ac0152048ef52723196c26ca5f2629dafb782aec1c66fc30531bf54335043
++R f512847bc3fbc8f671b8d43110416970
++T +closed 051ac0152048ef52723196c26ca5f2629dafb782aec1c66fc30531bf54335043
+ U dan
 -Z f721373f748dbb5b0d3c5c4f1daf5230
++Z 161902ccd44d5b310812ea403a55d987

diff --cc manifest.uuid
index 4bf71e72d893045fda861669b2a4d3b9eab8906e,ec1e91062fac5fb5dada2bfb7c66ca2c56b3797f..f49754486e603d5260c0d5c0e658bc23fcda93e0
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 529fb55e3d00472f13446117527b0896827b11e870b581af7fe7cbb7392ef3cd
 -051ac0152048ef52723196c26ca5f2629dafb782aec1c66fc30531bf54335043
++41399169954f9bef53c3fa89879f39823b80bd127f76cf60abbe24217878a571

diff --cc src/main.c
Simple merge

diff --cc src/wal.c
Simple merge