CCBC-389: Disarm timeout when no commands remain

Also:
- Make purge_single_server return the number of items purged
- Don't refresh bootstrap count unless something has actually failed
- Fixup log messages so users don't get alarmed. The invocation of the
  timeout callback itself is perfectly normal (and is more efficient
  than constantly rescheduling)

Change-Id: Ie52d18201711ce3b74f09b75fceb9706088450b6
Reviewed-on: http://review.couchbase.org/36242
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>