Merge branch '266-message-queues' into 'master'

Message queues. #266

A message queue is a group shared by two devices that delivers messages from each device to the other in order. The first 64 bits of the message body contain a sequence number that's incremented for each message sent in a given direction. The incoming and outgoing sequence numbers and information about any messages received out of order and waiting to be delivered are stored in the group metadata.

See merge request !121