Refactor protocol message parsing:

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.

Reviewers: @ximinez, @scottschurr 
You defined a local `streambuf`, but didn't use it throughout the function.
```
                (&e == &*buffers_->streambuf_->list_.begin() ?
                    buffers_->streambuf_->in_pos_ : 0);
```
-> 
```
                (&e == &*streambuf.list_.begin() ?
                    streambuf.in_pos_ : 0);
```