SPY-174: Make sure MultiOperationCallback is threadsafe

First, special thanks goes to @exortech for reporting and fixing the issue
here: https://github.com/dustin/java-memcached-client/pull/9

Motivation
----------
Certain operations like the multi get callback rely on the multi operation
callback which has a non-thread safe component to it.

Modification
------------
Make the callback atomic and thread safe.

Result
------
No race conditions with bulk get operation callbacks.

Change-Id: I511a8e5ec6e8fe50168337a4b9bbddf2360bd365
Reviewed-on: http://review.couchbase.org/38358
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>