connection: keep names locked while sending messages

connection: keep names locked while sending messages

If we send messages and we look up well-known names, we must keep them
locked until we're done. Otherwise, the name might move to another
connection before we queue it on the resolved name.

This fixes the sending path to lock names for the entire function call.
Name-movements already lock the name-registry for the entire move, so we
should be fine now.

We also inline the lookup-helper, as it is only used once and will get
messy if we return kdbus_name_entry objects.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>