JCBC-373: The replica get future to complete is not thread safe.

This leads to false positives while the future could be completed
but never is because of visibility issues.

Change-Id: I28a74e3f44d28ab8e4f3780ddb158527484fd59d
Reviewed-on: http://review.couchbase.org/29823
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>