MB-11217:Use getShard() to retrieve shard for a vbucket id

When a rollback request is received for a particular vbucket id
on UprConsumer, we need to use getShard() function to get the
shard for a vbucket id

Change-Id: I6e69dcb50313e3d46a8021638b7fd25e6225279a
Reviewed-on: http://review.couchbase.org/37677
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>