XD-554 Plugin Refactoring

Separate module context refresh() and start() to allow
post refresh/pre start configuration.

XD-554 Set up Channel Registry in Post Processing

Remove ChannelRegistrar and register channels during
post processing.

XD-554 Polishing (PR Comments)

Added ChannelRegistrationTests to verify the input and output
channels are correctly registered.

Ran it to verify it would have picked up the first PR comment.

XD-554 More Polish

Fix references to AbstractPlugin.setPostProcessContextPaths()