NCBC-552: Ensure that bucket throws ObjectDisposedException

When a thread calls Dispose on a bucket and no other thread is referencing
it, subsequent operations will fail with an ObjectDisposedException: "This
bucket [{0}] has been disposed! Performing operations on a disposed bucket
is not supported!"

Change-Id: Ib3c8d96a48a9e24c102133c8f37b37792acb150d
Reviewed-on: http://review.couchbase.org/39381
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Brett Lawson <brett19@gmail.com>