This moves the various copies of port forwarding code in the Device subclasses into the ProtocolDiscovery class.
* move port forwarding to a common location
* throw exception if protocol Uri is not discovered or port forwarding fails
* cancel discovery protocol subscriptions on iOS launches (wasn't happening before)
* fix iOS port forwarding to match other implementations
* add tests
* rename service_protocol.dart to protocol_discovery.dart
* add a wrapper around the obs. protocol
* use json-rpc in run
* consolidate obs. code; implement flutter run --benchmark
* review comments