New callback for generic message handling

[oliveigah]

As discussed in #46, introducing a new callback for generic message handling inside the pool process could be very useful!

It would enable at least two features:

• Idle termination of empty pools (currently not possible with handle_ping/2, as it only pings workers).

• Safe pool termination, avoiding race conditions when a checkout occurs just as the pool is about to terminate due to idleness.

From what I understand of the proposed solution, it would involve:

1. Adding a handle_info/2 clause to NimblePool that matches messages like {:pool_info, term}.

2. Adding pool_send/2 and pool_send_after/2 functions to send a term to the pool, wrapping it in the :pool_info tuple.

3. The new handle_info/2 would unwrap and forward the term to a user-defined handle_pool_info/2 callback, which would receive the term and the pool state.

4. The handle_pool_info/2 may return the same options as the GenServer's handle_info callback and it will in fact be the return value of the new handle_info/2

I think this approach is as flexible as we could get and may enable the user side implementation of pretty much everything.

If you're on board with this idea and implementation, I can probably start working on it by the weekend.

Let me know your thoughts! Thanks!