Replace WT_SESSION_NO_CACHE_CHECK with a more general-purpose flag,
WT_SESSION_LSM_TREE_LOCK, and set/clear it in the standard LSM tree
functions.

Fix a trivial bug where WT_SESSION_NO_SCHEMA_LOCK was left set in a
session if the LSM tree lock function failed.
Revert name change of flag to WT_SESSION_NO_CACHE_CHECK
Merge pull request #1077 from wiredtiger/lsm_lock_flag

LSM tree lock flag