Add option to wait for output of user event

This is just a simple feature request but one that I thought would be able to solve a lot of problems for us - the ability to have an optional parameter when sending a user event that will cause the client to pause until the event has fully processed and then return the output.

This obviously wouldn't be appropriate for all use cases but at the moment people are hacking together weird handlers for long running processes and it could be solved with an option in serf itself.

The issue does not name files, tests, or an entry point. Start by tracing how Serf sends user events and how clients receive event results, then clarify the waiting semantics, timeout behavior, and output contract. Done means an optional request setting can wait for processing and return its output without changing existing asynchronous use cases.

Written by the indexing model from the issue text.