feat: Reset request cache before handling event

[timmc-edx]

This is a just-in-case measure -- we don't know of any specific code that handles events and relies on the cache. But it's likely to come up at some point.

Ticket: https://github.com/openedx/openedx-events/issues/235

Merge checklist: Check off if complete or not applicable:

• [x] Version bumped
• [x] Changelog record added
• [x] Documentation updated (not only docstrings)
• [x] Commits are squashed
• [x] Noted any: Concerns, dependencies, deadlines, tickets, testing instructions