MB-11104 Synchronize reading the last seq num of each vbucket.

Reading the last seq num of each vbucket should be guarded
through a lock.

Change-Id: I63cf73b831a9479da39b27a0c03c4c0a8ea50ba1
Reviewed-on: http://review.couchbase.org/37464
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>