bc_file bugfixes

- Always update last_mtime after successful update
- Don't check vbucket changes. This is done by confmon and the core

Change-Id: I7d5b62149ca372ff8881f7673697adb9458f11b6
Reviewed-on: http://review.couchbase.org/34988
Reviewed-by: Subhashni Balakrishnan <b.subhashni@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>