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 
well... `ByteSize` is part of the protocol buffers api so I'm going with the flow here.