pymessaging: Add a hook to run the event loop, make callbacks practical

pymessaging: Add a hook to run the event loop, make callbacks practical

These change allow us to write a messaging server in python.

The previous ping_speed test did not actually test anything, so
we use .loop_once() to make it actually work.  To enable practial use
a context is supplied in the tuple with the callback, and the server_id
for the reply is not placed inside an additional tuple.

In order to get at the internal event context on which to loop, we
expose imessaging_context in messaging_internal.h and allow the python
bindings to use that header.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>