winbindd: Introduce "bool_dispatch_table"

This is meant to replace the synchronous "dispatch_table".

The current dispatch_table assumes that every synchronous function does
the request_ok or request_error itself. This mixes two concerns: Doing
the work and shipping the reply to the winbind client. This new dispatch
table will make it possible to centralize shipping the reply to the
client. At a later stage this will enable easier statistics on how long
request processing took precisely.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
