Fix some issues with the recent changes to handle locking:

1) bump the reference count before dropping the handle list lock.  Otherwise, a handle sweep could free the handle from under us in between.

2) hold the handle exclusive until sweep is finished with it.  Previously it could have been re-acquired in between, and the locking situation was unclear.

refs #1391, #1392
Merge pull request #1400 from wiredtiger/handle-locking-fixes

Fix some issues with the recent changes to handle locking