NCBC-634: Ensure that CancellationTokenSource is Disposed

This fix ensures that the CTS will be explicitly disposed so that the
object does not live through to finalization.

Change-Id: I39545b97a3c934f2c0a93587fa7147b32d2bd043
Reviewed-on: http://review.couchbase.org/41335
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Brett Lawson <brett19@gmail.com>