[Refactor] Make Peerseeding connection agnostic

[matsjj]

This finishes the move completely away from ConnectionManager doing business logic against the running connections and being responsible only for building and maintaining connections as necessary.

I suppose the easiest for now is to follow the same approach did with ChannelOpener, ChannelCloser and SyncClient interfaces, and a central listener that issues the command to ask for seeds.

In the end we don't have to cancel and rebuild connections for each step, and we can reuse existing connections to seed, sync and build payment channels after that.