Process announcement_signatures messages in Channel and store sigs

Process announcement_signatures messages in Channel and store sigs

Previously we handled most of the logic of announcement_signatures
in ChannelManager, rather than Channel. This is somewhat unique as
far as our message processing goes, but it also avoided having to
pass the node_secret in to the Channel.

Eventually, we'll move the node_secret behind the signer anyway, so
there isn't much reason for this, and storing the
announcement_signatures-provided signatures in the Channel allows
us to recreate the channel_announcement later for rebroadcast,
which may be useful.