source4 smdb rpc_server: Support prefork process model.

source4 smdb rpc_server: Support prefork process model.

Allow the rpc_server to run in the prefork process model. Due to the use
of shared handles and resources all of the rpc end points are serviced
in the first worker process. Those end points that can be run in
multiple processes (currently only Netlogon and management) are serviced in
the first and any subsequent workers.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>