ldb: Add ldb_handle_get_event_context()

ldb: Add ldb_handle_get_event_context()

This will allow us to obtain a private event context for use while we hold
locks in ldb_tdb, that is not shared with the global state of the application.

This will ensure we do not perform other operations while we hold the lock

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>