Reset RequestCache between message processing

[robrap]

A/C:

• Clear RequestCache between processing messages
  • feat: Reset request cache before handling event event-bus-kafka#201
• Update how-to for concrete implementation.

Notes:

The RequestCache should be cleared between processing of each message to treat message processing like a request. In case any code is shared between message processing and the service itself, the RequestCache should have a common interpretation. Basically, we will be replacing this call that is currently in the RequestCacheMiddleware.

Note that for now the only cache between messages would be the shared Django cache (e.g. Memcached). We do not support a consumer level cache, and will leave that for when and if it becomes a priority.