Cancellable Operations

[ajtack]

Currently, a get/put/delete operation must be waited upon at the client.hxx level. It so happens it may be easy to shut down networking and thus trigger a rapid flood of socket cancellation errors, but this seems a bit roundabout.

It may be valuable to be able to cancel an operation at the client.hxx interface level. This also gets back into synchronous programming against Riak-Cpp, though, which I'm inclined to discourage.

This idea is in the "Do we need it?" question phase.