Support logging remote timestamps on third-party node

[AustinSchuh]

If we have a system with nodes a, b, and logger, and are only running a logger on logger, then currently it is not possible to be log timestamps for messages sent between a and b. So if there is a channel that is forwarded from a to b and the logger, then if you attempt to replay a log from the perspective of b you will not get any messages replayed on that channel since the logger didn't log the timestamps for the message arrivals on b, so doesn't know when the messages actually arrived :(.

This means being able to specify arbitrary nodes in the timestamp_logger_nodes field for a connection, see:

aos/aos/configuration.fbs

Lines 28 to 32 in e7c7e58

	// If the corresponding delivery timestamps for this channel are logged
	// remotely, which node should be responsible for logging the data. Note:
	// for now, this can only be the source node. Empty implies the node this
	// connection is connecting to (i.e. name).
	timestamp_logger_nodes:[string] (id: 2);