Grab the lock before registering seq-based replication cursors.

Change-Id: I8bed8da1f222ccc1d04bc22ee64951cf09dd9fc5
Reviewed-on: http://review.couchbase.org/34388
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>