refactor command RPCs.

[raulk]

This pull request proposes a few minor protocol changes at the RPC level.

• Renames and new messages.
  • ClientSignal is renamed to ClientCommand.
  • We introduce a CommandResponse message for protocol completeness.
  • We introduce a ServerNotice message so that the server can signal circumstances to the client, e.g. that retained messages are being dropped.
• The configuration is now expressed in Protobuf, and not in embedded JSON any longer.
• The CONFIG_EMITTER command is replaced by an UPDATE_CONFIG command.
  • The client sends the requested configuration in the config field.
  • The server will respond with a CommandResponse including the effective configuration.
• We introduce a HELLO command that the client must greet the server with upon establishing a WS connection. It can optionally send a config object.
• We introduce a CommandResponse object with which the server returns the result of a command.
• We introduce unique command IDs so we can correlate requests and responses (we don't expect parallelism right now).
• For completeness, we introduce a STOP_PUSH command.