Defer computing the number of bytes of free space on a btree page until that

value is actually needed.

FossilOrigin-Name: 177f5f40eabfcfd229ac7f291dfed9e9ee35762e86923a0f356915f389da177d

diff --cc manifest
index 7576156bdacf0d7009547cd61718d34887f5a759,cbed7e3fdec9978ba364500e8ea2ac8ff7b9dbca..e7da767f3977e998db0b3e116bddb0fd14b5cb10
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Fix\sanother\ssegfault\sthat\scould\soccur\sin\sfts5\swith\sa\scorrupted\sdatabase.
- D 2019-02-11T16:12:09.807
 -C Change\san\sassert()\sinto\sa\sNEVER(),\ssince\sthe\scondition\sis\sdifficult\sto\sprove\nwith\scertainty.\s\sImproved\scomment\son\sthe\sMemPage.nFree\sfield.
 -D 2019-02-12T00:58:10.125
++C Defer\scomputing\sthe\snumber\sof\sbytes\sof\sfree\sspace\son\sa\sbtree\spage\suntil\sthat\nvalue\sis\sactually\sneeded.
++D 2019-02-12T01:04:49.403
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 178d8eb6840771149cee40b322d1b3be30d330198c522c903c1b66fb5a1bfca4
@@@ -455,10 -455,10 +455,10 @@@ F src/auth.c 0fac71038875693a937e506bce
  F src/backup.c 78d3cecfbe28230a3a9a1793e2ead609f469be43e8f486ca996006be551857ab
  F src/bitvec.c 17ea48eff8ba979f1f5b04cc484c7bb2be632f33
  F src/btmutex.c 8acc2f464ee76324bf13310df5692a262b801808984c1b79defb2503bbafadb6
- F src/btree.c 18046bf14f0e3fa294ef3f7c2dc30ca7e95f3ac11ec222ad906e40b150051bde
+ F src/btree.c 7457f7813873877041ec7e04d76a041c81831704a73419882fbe1c222ce6a68c
  F src/btree.h 63b94fb38ce571c15eb6a3661815561b501d23d5948b2d1e951fbd7a2d04e8d3
- F src/btreeInt.h cd82f0f08886078bf99b29e1a7045960b1ca5d9d5829c38607e1299c508eaf00
+ F src/btreeInt.h 6111c15868b90669f79081039d19e7ea8674013f907710baa3c814dc3f8bfd3f
 -F src/build.c 906ca6663b9dcd413e72ae9c44dd51e596d8336b04d52e678a7501e71c20cab2
 +F src/build.c b0a9ee5b551afbc8357a68eb30693973300daf845c8c0e564f672d9b3fdeec56
  F src/callback.c 25dda5e1c2334a367b94a64077b1d06b2553369f616261ca6783c48bcb6bda73
  F src/complete.c a3634ab1e687055cd002e11b8f43eb75c17da23e
  F src/ctime.c 109e58d00f62e8e71ee1eb5944ac18b90171c928ab2e082e058056e1137cc20b
@@@ -1804,7 -1804,7 +1804,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P c19e97bc927074915a3ac8307bf89d30580cf90edd75f4001bcf589b2dfefe58
- R ba860dbc894a4065c86c011d6f21e82a
- U dan
- Z 42d44b957f080a8e59025584ead66359
 -P f11b0ed47402034378ded2c60b82841b81cd1319cdb72d1ef052faef29a5294d
 -R b9fd86c9b0e983ebe73f0426098d02c6
++P 09e337386beb2008eba969f79601d1b8ed953951ce6e43782586e168140d78d2 fec071b89df2cbafda365077bb54bc9999e12086f36d6cea8d125662b36638dc
++R 49e3547600599943d74a3169c60af8d6
++T +closed fec071b89df2cbafda365077bb54bc9999e12086f36d6cea8d125662b36638dc
+ U drh
 -Z 3b166eeca4670760101eb14216736649
++Z 7aab7a7f295f8d3f5fcc30e3cc343e77

diff --cc manifest.uuid
index a02693386db1a510b89efd2f253c22b3606b8c3e,de8af99627865bb92a010279f26f1ed3cc6d4a3d..1f83fd7926c006f6f26972d33647bd1f592326f9
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 09e337386beb2008eba969f79601d1b8ed953951ce6e43782586e168140d78d2
 -fec071b89df2cbafda365077bb54bc9999e12086f36d6cea8d125662b36638dc
++177f5f40eabfcfd229ac7f291dfed9e9ee35762e86923a0f356915f389da177d