feat(network): add methods for requesting arbitrary rpc to ConnectionPool

`PoolWorker` is majorly reworked and now supports executing various
requests asynchronously. The caller is notified of the request's outcome
via callback. The connection is dropped on error only if there are no
requests awaiting response (or timeout).

`ConnectionPool` now has
- methods `call_and_wait`/`call_and_wait_timeout` which yield
- method `call` which doesn't yield but accepts a callback