refactor(peer) some _minor_ safety features

This PR cleans up the peer package, making it a tiny bit safer to use.

* All peers are created using Peerstore. It is no longer simple to instantiate a naked peer. To create naked peers, use the `mockpeer` package.

(This refactor was performed because of work #416 and a promise I made a long time ago)
the merging is no longer there because we always create it here. sure, it's a bit of noise. can rebase to move the ds stuff to one commit so it can be reverted easily?