Syncing MemcachedClientIF with MemcachedClient.

This changeset makes sure all implemented methods are available
in the interface. Also, @Override annotations are added for
clarity.

Change-Id: Iedb899ed9d70a5d76e7d3891524336a21edf34b8
Reviewed-on: http://review.couchbase.org/31669
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>