We now attempt to send all queued messages per call to cqueue_try_send() instead
of just the oldest message in the queue. This speeds things up substantially.
Also fixed a very unlikely bug where the read receipt might wrap around to zero
which we used as a reserved value for an unsent message
This fixes a bug where lines would sometimes be incorrectly marked as unread, as well as
a bug where inbound messages would sometimes be coloured incorrectly