Rewrite S2BT to avoid lots of "dereference of a null pointer" warnings from clang-analyzer.
Cleanup: __wt_update_check() has lost most of its functionality over
time, replace it with calls to __wt_txn_update_check, and remove the
calls entirely when there's no WT_UPDATE structure and so no possibility
of detecting a collision.
Remove "Memrata" doc link from the top-level navigation.
Cut release 1.6.4
Bump release version on develop branch to 1.6.5
Fix some clang "dead assignment" warnings.
Merge pull request #632 from wiredtiger/clang-analyzer-warnings

Fix some clang-analyzer warnings