messaging3: Add messaging_read_send/recv

messaging3: Add messaging_read_send/recv

This is made to replace the msg_channel abstraction.

msg_channel was created to not miss any messages. For this, some
complex queueing was installed. This complexity has caused quite a
few problems in the past (see bug 10284 for example).

messaging_read_send/recv is able to achieve the same goal with a
lot less complexity. The messaging_read_send atomically installs
the reader into the messaging_context, we will not miss any messages
while this installed. messaging_send_recv will deinstall that
listener, but in the callback function you can directly call
messaging_read_send again without going through the tevent_loop_once.
As long as this is always made sure, no messages will be lost.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>