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 PeerImp objects that exist
represent active peer connections that have already gone through the
handshake process.
This replaces the stateful class parser with a stateless free function.
The protocol buffer message is parsed using a ZeroCopyInputStream.
* Invoke method is now a free function.
* Protocol handler doesn't need to derive from an abstract interface
* Only up to one message is processed at a time by the invoker.
* Remove error_code return from the handler's message processing functions.
* Add ZeroCopyInputStream implementation that wraps a BufferSequence.
* Free function parses up to one protocol message and calls the handler.
* Message type and size can be calculated from an iterator
range or a buffer sequence.
