IRPC not invoked in prefork master

[Pavel Kalugin]

Hi. I added IRPC function to the ldap server and noticed it gets never 
called. Digging it I found what I believe is a bug in the messaging 
code: when prefork master receives imessaging datagram targeted to the 
child messaging context it dispatches it to the child tevent, but there 
is no event loop running on that. The result is that IRPC is not called 
and both the dgram and `tevent_immediate` are leaked (i.e. they sit in 
the queues forever).

The fix that is working for me is here: 
https://gitlab.com/samba-team/samba/-/merge_requests/3257

I would appreciate it if someone would take a look at this. My 
understanding of the subject is limited and probably there may exists a 
more correct solution, or, rather, I'm doing something wrong with IRPC.


-- 
Pavel Kalugin
inno.tech