Performance improvements when reading large blobs, especially if

SQLITE_DIRECT_OVERFLOW_READ is defined.

FossilOrigin-Name: 2312eb6a9eb31051db4e0baf19e033ba39adc7b1

diff --cc manifest
index d36a69418adb61001f3e70c88579add23a5fae7c,dbe10602d3f6660db5b16285d0423e7d710cb9e5..51582695c0c6efbc3adc309035c09165716e9f9f
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Merge\schanges\sto\sthe\squery\splanner\sthat\sstrive\sto\sensure\sthat\sany\sindex\s\nusage\sthat\sis\sa\sproper\ssubset\sof\ssome\sother\sindex\susage\salways\shas\sa\sslightly\nhigher\scost.
- D 2014-04-04T18:20:35.620
 -C Remove\san\sunnecessary\sconditional,\sreplacing\sit\swith\san\sassert().\nImprovements\sto\scomments.
 -D 2014-04-01T01:24:34.512
++C Performance\simprovements\swhen\sreading\slarge\sblobs,\sespecially\sif\nSQLITE_DIRECT_OVERFLOW_READ\sis\sdefined.
++D 2014-04-04T18:49:19.417
  F Makefile.arm-wince-mingw32ce-gcc d6df77f1f48d690bd73162294bbba7f59507c72f
  F Makefile.in 2ef13430cd359f7b361bb863504e227b25cc7f81
  F Makefile.linux-gcc 91d710bdc4998cb015f39edf3cb314ec4f4d7e23
@@@ -1160,8 -1159,7 +1160,8 @@@ F tool/vdbe_profile.tcl 67746953071a9f8
  F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
  F tool/warnings.sh d1a6de74685f360ab718efda6265994b99bbea01
  F tool/win/sqlite.vsix 030f3eeaf2cb811a3692ab9c14d021a75ce41fff
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- R dcb8d4aedf5f44fc76078dc97c08f05d
- T +closed 7473c4dfc10a47594affa6f4e071a08dc8838c0c
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 -R 193f537069a669cf49d79f97b46daa7e
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++R 3de83004edd963c4959934c7f5a61b03
++T +closed 834bf1c367e1ccd498c7f9f843be2d2aa11ffb3c
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++Z f4df6ef034f1a63bb656a3ddc69e0011

diff --cc manifest.uuid
index 8da78e64c345dba0838a009d0e614138c2cd8cc8,44d48365ac85744adf3a079b0c652e9f5a3aa9ba..0466f25ed2079df34b61f6d8c07ce2488efbf2cd
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 683dd379a293b2f330e1e4cd746f190527fe48ee
 -834bf1c367e1ccd498c7f9f843be2d2aa11ffb3c
++2312eb6a9eb31051db4e0baf19e033ba39adc7b1