MB-10510: Persist failover logs to handle rollback case

At the same time, need to handle multiple buckets case with the
following db file directory structure:
backuproot \
  	bucket-1 \
  	bucket-2 \
  	bucket-3

Change-Id: Icb291ed33d4bac73e1f8e56fdebf1a4be2454b13
Reviewed-on: http://review.couchbase.org/35027
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>