This refactors the heartbeat payload into an event payload, of which
a heartbeat is one of the possible options. This also refactors the
Concentratord ZMQ interface, moving away from multipart messages in
favor of a single message containing an Event / Command Protobuf
message.

The events feature makes it possible to periodically send (proprietary)
events from the Relay gateways.

The commands feature makes it possible to send commands to the Relay
gateways, in which case the output is returned as an event.

This implements the encryption of event and command payloads, to avoid
the exposure of possible sensitive data (e.g. configuration parameters).

This also adds a new root_key configuration, which will be used to derrive
the keys for mic calculation and encryption. In case the signing_key
configuration is present, then this key will be used for mic calculation
(for backwards compatibility).

This makes it possible to override the relay_id instead of
(automatically) using the 4 least significant bytes of the Gateway ID.