Open up EncoderChannel and DecoderChannel for extension.

We need our subclasses of EncoderChannel and DecoderChannel to access
the def_registry field when using Firefly in order to short-circuit the
registration of Firefly's protocol types.

We also need access to the writer field from one of our subclasses of
EncoderChannel since we collect the data from one encoded sample and
flush the writer when it's done, placing the encoded data into a Firefly
data sample which is sent on the wire.