s4-messaging: fixed the removal of messaging sockets in child tasks

s4-messaging: fixed the removal of messaging sockets in child tasks

when a child task exits we were firing a destructor on any inherited
messaging contexts, which could trigger a removal of the parents
message socket and messaging database entry.

This adds a new auto_remove flag to imessaging_init(), and exposes the
cleanup code for use by the stream service.

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Fri Jul 22 08:09:06 CEST 2011 on sn-devel-104