Refactoring protocol packages

[andrewwhitehead]

As discussed with @swcurran

• Protocol packages (currently located under aries_cloudagent.messaging along with various messaging framework classes) should be moved into a new subpackage, likely aries_cloudagent.protocols. This aligns with the move from 'message families' to 'protocols' in the RFCs. It may be desirable to use a script or refactoring tool to reduce the manual edits required as a result.
• In order to improve the generated documentation, class files should be moved out of the base package (including classloader.py, conductor.py, dispatcher.py, and friends) into existing or new subpackages.

This change should be aligned with a minor release to allow any dependent packages to be updated.

[swcurran]

Let me know when you do this as I've put links in the RTD index.rst to the protocols generated code. Best to align this change with an update to the index.rst.

[andrewwhitehead]

Addressed by #250, #301