Implement hooks as async functions

Hooks can actually mutate or remove a document.   When they do so, they should call into the cluster in the typical async fashion.   Doing so, they would need to be passed a callback to call when done.   

For now, I have a workaround where we spawn a thread, call the cluster operation and return.  Then, the hook monitors the future (with wait(0), and a this_thread::sleep_for) until it is done.   Ugly, but ok for now.   Lets redo this, and so they hooks can be called as if they are (at least potentially) async, with a callback.