NCBC-369: ObjectDisposedException is not handled in PerformMultiGet

Change-Id: I4576efb24e07070d0547eb4a25e6e394afd803a0
Reviewed-on: http://review.couchbase.org/32392
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>