Add reliability tests for direct-to-device messaging

[NegativeMjark]

Adds tests for matrix-org/synapse#1046

[leonerd]

A bold choice of name there. Maybe send_message_to_device; or perhaps is the "message" part implied? Could it be just send_to_device ?

[leonerd]

Having got to the end I now see that to device message is in fact a compound noun, not part of the verb structure. I wonder if this code would be a bit more readable were these renamed to something else, perhaps a "device-directed message" or similar? If that name sounds a bit long, I don't think it would be unreasonable to document/specify these at the protocol level and introduce the initialism DDM to refer to it; then we can name functions

sub matrix_send_DDM
sub matrix_recv_DDM
sub matrix_recv_and_ack_DDM
etc...

[NegativeMjark]

@leonerd I've called it a device_message for now since DDM is probably a bit meaningless for someone trying to read the code.

sub matrix_send_device_message
sub matrix_recv_device_message
sub matrix_recv_and_ack_device_message

[leonerd]

LGTM