Add in-memory message queue

[yxjiang]

The problem

Currently we are using a simple python queue.Queue as the message queue. It is inconvenient to share it across components when the program grows more complicated. Ideally we want to have a message queue that can run in an independent process with an exposed endpoint.

The potential solution

There are many message queues we can leverage, and we prefer to use an lightweight one. Redis pub/sub could be a good choice.