Watchdog with profiling

[arepetti]

Implements #50

• A better watchdog that attempts to assess the system health monitoring runners.
• It optionally exposes a gRPC service to query the system status (for example to display it in a web dashboard).
• GrpcHost transcodes gRPC calls to JSON (by default, but you have to enable it per method call using annotations).
• Service discovery should be slightly faster: the supervisor makes the discovery service address available in an environment variable (as soon as known) then runners loaded after it should have it without a call via a named pipe.