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
Add a new WT_SESSION::transaction_range_pin method.

Can be used by applications to identify sessions that are keeping
transaction IDs pinned.
Update transaction_range_pinned as per review comments.
Use TXNID_LT in preference to WT_MIN
Fix transaction ID logic (again), add session method entry / exit, add a documentation snippet.
Merge pull request #1314 from wiredtiger/transaction-range-pin

Add a new WT_SESSION::transaction_range_pin method.
Add new transaction_pinned_range to NEWS file.
Merge pull request #1310 from wiredtiger/mongo-fixes

Enhancements for MongoDB
Coverity CID 1249673: Structurally dead code  (UNREACHABLE)
Push down the force-discard logic so we don't have to deal with already-closed trees.
Grammar fix.
Merge branch 'develop' into readwrite-locks