Addings helper methods to HTTP/2 handler

Motivation:

Subclasses of AbstractHttp2ConnectionHandler have to implement all frame
handler methods, many of which can be ignored in many cases.  Also there
is no easy way to access the connection object.

Modifications:

Added default implementations for frame handler methods to
AbstractHttp2ConnectionHandler, and added an accessor for the
connection.

Also fixed example test for HTTP/2 with cleartext upgrade. It must have
been broken by recent commits.

Result:

AbstractHttp2ConnectionHandler is more subclass-friendly.
None of these buffers are retained, just sliced from the original.  Since AbstractHttp2ConnectionHandler is a BytesToMessageDecoder, do we have to release the buffer?

Also, our docs for Http2FrameObserver suggest that if the buffer needs to be retained, it must be copied: https://github.com/netty/netty/blob/master/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2FrameObserver.java