Add statistics to track open sessions and the range of transaction IDs that are pinned.
Close chunk cursors on LSM reset, so that nothing stays pinned in cache.
Forcefully discard a tree from cache if doing a forced drop.
../src/lsm/lsm_cursor.c:115:13: variable 'c' set but not used
Merge pull request #1310 from wiredtiger/mongo-fixes

Enhancements for MongoDB