Update go-couchbase and gomemcached to get fix for deadlock issue

couchbase.Bucket.Update and db.sequenceAllocator got into a deadlock
due to the allocator's making a bucket request from within an Update
callback, while Update was already holding open a socket connection.
If enough of these happened at once the limited pool of sockets could
run out, and the sequenceAllocator would never be able to send its
request.
Fixes #119. (I think; unit tests pass & replication to Couchbase Server
works, but I'm waiting on Tommie to re-run the large scale test that
uncovered the issue.)