Split peer connect logic to another class (RIPD-711): 

All of the logic for establishing an outbound peer connection including the initial HTTP handshake exchange is moved into a separate class. This allows `PeerImp` to have a strong invariant: All objects that exist represent active peer connections that have already gone through the handshake process.

Reviewers: @nbougalis, @rec 
"buffer sequence" is specific boost::asio terminology. Elements of the sequence do not have to be `const_buffer`.