If an exclusive operation such as verify conflicts with a sweep, wait.  Normally, exclusive operations don't conflict on the data handle, they wait on the schema lock.

refs #1309
Clarify the difference between handle locking in session_dhandle.c vs conn_dhandle.c (briefly: the schema lock).
whitespace.
Implement Keith's review feedback.

refs #1309
Add a big honkin' comment about handle locking.
Merge pull request #1336 from wiredtiger/dhandle-exclusive

Fix conflicts between sweep and exclusive operations