VBucketBackupWriter should keep track of the last committed seqnos on a per-vbucket basis

At the moment, we have a single variable to keep track of the last committed seqno across the VBucketBackupWriter; this is invalid because we keep track of persisted data on a per-vbucket level. We should maintain a LCS for each vBucket and only persist the snapshot if the LCS has advanced for that vBucket.