Make tunnel resilient to hanging network sockets

[boronine]

The test system introduced by the latest commit allows us to emulate bad network conditions by passing the tunnel through our well-tested "NetworkEmulator" class, which can be used to pause and unpause sockets. This kind of problem requires a timeout of sorts, perhaps native TCP keepalive, perhaps a regular HTTP2 ping failing to receive which will prompt us to destroy the socket and attempt reconnection until network is back.

[boronine]

gRPC uses HTTP2 ping https://grpc.io/docs/guides/keepalive/