Update RemoteDispatcher to use BlueskyConsumer

[jklynch]

The RemoteDispatcher class should make use of the BlueskyConsumer class rather than duplicating its code. This PR modifies the RemoteDispatcher to delegate to a BlueskyConsumer.

Other changes include:

• new test fixtures added
• tests updated
• the default serialization for BlueskyConsumer has been changed to MessagePack
• commit_on_stop_doc in BlueskyConsumer has been replaced by a new consumer parameter to the process_document function allowing clients that do not subclass BlueskyConsumer to define their own policy for Kafka consumer commits
  • docstrings have been added and updated

[jklynch]

The CI failure is maybe (still) related to mongo? I think Garrett's latest PR to mongobox will fix this.