FO: Use tevent_req_defer_callback() when notifying callers
If a fo_resolve_service callback would modify the server->common member
in any way, for example by dereferencing the server and lowering the
refcount to 0, which would free the common structure, then the next
iteration of fo_resolve_service_done would access memory that was
already gone.
Please see
https://tevent.samba.org/group__tevent__request.html#ga09373077d0b39e321a196a86bfebf280
for more details.
Reviewed-by: Pavel Březina <pbrezina@redhat.com>