The logic of dispatching the announcement_signatures message was
distributed over several places and daemons. This aims to simplify it
by moving it all into `channeld`, making peer_control only report
announcement depth to `channeld`, which then takes care of the
rest. We also do not reuse the funding_locked tx watcher since it is
easier to just fire off a new watcher with the specific purpose of
waiting for the announcement_depth.
Signed-off-by: Christian Decker <decker.christian@gmail.com>