Refactor PeerFinder:
* Revise documentation in README.md
* Inject abstract_clock in Manager
* Introduce the Slot object as a replacement for Peer
* New bullet-proof method for slot accounting
* Replace Peer with Slot for tracking connections
* Prevent duplicate outbound connection attempts
* Improved connection and bootstrap business logic
* Refactor PeerImp, PeersImp private interfaces
* Give PeersImp access to the PeerImp interface
* Handle errors retrieving endpoints from asio sockets
* Use weak_ptr to manage PeerImp lifetime
* Better handling of socket closure in PeerImp
* Improve the orderly shutdown logic of PeersImp