Async response handler

Use sequence numbers in opaque to find callbacks rather than rely on
ordering. In general, memcached will respond in order, but this way (1)
it fixes the bug when performing several commands before socket
connection has completed and (2) we're not *reliant* on ordering.
We probably don't need profanity in the tests after all
Merge pull request #22 from alevy/lazyconn

Async response handler