lib: tevent: Initial checkin of threaded tevent context calling code.

lib: tevent: Initial checkin of threaded tevent context calling code.

Adds 2 new functions:

struct tevent_thread_proxy *tevent_thread_proxy_create(
                struct tevent_context *dest_ev_ctx);

void tevent_thread_proxy_schedule(struct tevent_thread_proxy *tp,
		struct tevent_immediate **pp_im,
		tevent_immediate_handler_t handler,
		void *pp_private_data);

Brief doc included. Tests, docs and tutorial to follow.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11771

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 49bddd8e4756ef52b05b850aec4864749fcf31cb)