Avoid EBUSY returns to verify and salvage caused by checkpoints.  The "fix" involves blocking checkpoints while salvage or verify are in progress.

refs #1404, SERVER-16457
Revert some debugging changes.
Add comments before getting the checkpoint lock.
Merge pull request #1508 from wiredtiger/avoid-checkpoint-ebusy

Avoid EBUSY returns to verify and salvage caused by checkpoints