handle deferrred messages with an inproc socket

In the API and module flux_t handle implementations, we had a zlist of messages that we kept when we needed to do a synchronous RPC and something other than the RPC's reply came in.  Then at every point where we were returning to the reactor we processed that list.   Issue #81 described a situation where an item was added to that list before the zloop was entered, and another event was needed to unblock the zloop before it would be processed.

This pull request uses an inproc socket instead of a zlist for the deferred requests, and wires it to the zloop so that any messages in the queue will cause the zloop to become ready.  We get to remove all the hooks for processing the queue, and it fixes the case mentioned above.

Fixes #81