feat(network): arbitrary sync/async rpc requests

Needs !249 (merged)

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