Make ChannelKeys an API and template Channel with it.

Make ChannelKeys an API and template Channel with it.

Instead of having in-memory access to the list of private keys
associated with a channel, we should have a generic API which
allows us to request signing, allowing the user to store private
keys any way they like.

The first step is the (rather mechanical) process of templating
the entire tree of ChannelManager -> Channel impls by the
key-providing type. In a later commit we should expose only public
keys where possible.