DoH: Modularize main event loop

* Connection now provides HTTP/3.
* Network has the logic for resolving DNS and maintaining a Connection.
* Dispatcher routes requests to the appropriate Network or creates one
  if needed.
* IO and maintenance is performed via tasks rather than manually pushing
  the futures in the main event loop.

Bug: 202081046
Test: resolv_integration_test
Test: resolv_stress_test + I682678b84b35c575a3eb88c2c1c67aefd195616c
Change-Id: I4296d0c7a7852951f41418b18686794d8df781bd