Move merge of our features into the feature manager, rather than
updating our node announcement then retrospectively setting the
feature manger's set to the new vector. This allows us to use the
feature vector as our single source of truth for announcements.
In preparation for a more complex function signature for set node
announcement, separate get and set so that readonly callers don't need
to handle the extra arguments.
In this commit, we add a new sub-system, then `HostAnnouncer` which
allows a users without a static IP address to ensure that lnd always
announces the most up to date address based on a domain name. A new
command line flag `--external-hosts` has been added which allows a user
to specify one or most hosts that should be periodically resolved to
update any advertised IPs the node has.
Fixes#1624.
