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.