Merging HTTP/2 preface logic into connection handler

Motivation:

The HTTP/2 connection preface logic is currently handled in two places.
Reading/writing the client preface string is handled by
Http2PrefaceHandler while the reading/writing of the initial settings
frame is handled by AbstractHttp2ConnectionHandler. Given that their
isn't much code in Http2PrefaceHandler, it makes sense to just merge it
into the preface handling logic of AbstractHttp2ConnectionHandler. This
will also make configuring the pipeline simpler for HTTP/2.

Modifications:

Removed Http2PrefaceHandler and added it's logic to
AbstractHttp2ConnectionHandler. Updated other classes depending on
Http2PrefaceHandler.

Result:

All of the HTTP/2 connection preface processing logic is now in one
place.