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 
Could you make the capture explicit, please?  I think you're only capturing `&this` so it should be no big deal but after reading through all this code to make sure we didn't have dangling references, explicit capture is IMHO the way to go.