Add utility for resetting consumer Django state before each event

[timmc-edx]

Both event-bus-kafka and event-bus-redis call a _reconnect_to_db_if_needed function before processing each event. This ensures that the database connection is in a good state before work begins. In a normal request-driven environment, this would automatically be handled by Django.

We're also adding a call to clear the request cache in event-bus-kafka, in case there are signal receivers that would normally be called in a request context and that rely on the request cache.

openedx-events should probably host a utility function that can be called from each implementation's consume loop in order to generically reset the application state for each new event that needs to be processed. (Also see #236 for the possibility of more general request-like resetting.)

[robrap]

Just making it extra clear that "clear the request cache" still needs to be added to event-bus-redis, and this ticket would be a great way to get this change and future changes as well. Of course, with this change, someone would also want to retrofit event-bus-kafka to use the shared implementation.