PeerFinder work and refactoring:

 * Implement PeerFinder business logic.
 * Support fixed peers (including DNS support).
 * Add journal support to Peer and Peers.
 * Refactor PeerDoor support.
 * Tidy up Peers and eliminate connection functionality and timers.
 * Refactor Peer interface and add journal support.
 * Allow construction of incoming Peer using an existing socket.
 * Remove TESTNET support.
 * Misc. cleanups.