Make elli callback reconfigurable

Any running acceptor processes must be recycled before the new config actually takes effect, which can take a while due to the keep-alive loop.

Some options for updating the acceptors' config, none of which are very clean to me

* Keep callback config in an ets table
* Send new config to all acceptors which would have to receive the new config in the loop
* Forcefully recycle all acceptor processes, breaking any running requests