Sweep old handles more aggressively:

1. don't have checkpoint or other periodic operations like statistics logging keep old handles alive;
2. don't wait for all sessions to empty the file from their cache before closing;
3. only update the time of death from the sweep thread.
Merge branch 'develop' into sweep-more
The "inuse" field protects the btree, the "session_ref" field protects the dhandle.  Unlock the handle if we don't attempt to discard it, or if the discard fails.
Merge branch 'develop' into sweep-more
Close the right handle during sweeps.
Merge pull request #1456 from wiredtiger/sweep-more

Allow sweep to reclaim more handles