Request/response serialization should be done with JSON, not pickle

[chriskuehl]

pickle is a poor choice; code differences across the frontend and backend will lead to problems (they are not necessarily updated at the same time). In general pickle outside of the current process is a bad idea.

We're serializing namedtuples which consist entirely of strings/lists/booleans/simple primitives, switching to JSON should be trivial. Plus celery will stop printing a giant warning on start.

[matthew-mcallister]

This is blocking upgrading celery.

[chriskuehl]

celery 4 allows ssl for redis, so I'm going to re-enable pickle (it wasn't removed in celery 4, just disabled by default) so we can upgrade. We should definitely still do this ticket, though.