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
Never sweep the metadata.  We don't go through normal handle locking paths, and threads are not prepared to reopen the metadata.  Just skip it.
Track whether eviction is making progress regardless of whether the cache is 100% full.  Otherwise we can get into a tight loop.  Use the count of pages evicted rather than a flag, now that there are multiple eviction threads.
Convert size_t to uint32_t for log records.

refs #1359
Merge branch 'develop' into overflow-relax