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.
It's in Http2FrameObserver because that's the interface used by Http2FrameReader.  AbstractHttp2ConnectionHandler internally handles these events from the reader, updates its internal state and accepts/rejects the frame.  Once a frame is accepted, it forwards it to its own handler method for subclass-specific processing.  This is why AbstractHttp2ConnectionHandler implements Http2FrameObserver.  We could optionally not implement the interface and duplicate the methods it needs, making them protected.