SPDY: refactor frame codec implementation

Motivation:

Currently, the SPDY frame encoding and decoding code is based upon
the ChannelHandler abstraction. This requires maintaining multiple
versions for 3.x and 4.x (and possibly 5.x moving forward).

Modifications:

The SPDY frame encoding and decoding code is separated from the
ChannelHandler and SpdyFrame abstractions. Also test coverage is
improved.

Result:

SpdyFrameCodec now implements the ChannelHandler abstraction and is
responsible for creating and handling SpdyFrame objects.