Extract the key generation logic for events from RedisHLL

Problem

Currently, the logic how to create keys for events for a specific day is encapsulated within the implementation of RedisHLL based event collection. This logic should be refactored into a separate place in order to make it reusable between RedisHLL and Redis based event collection (see #411264 (closed))

Desired Outcome

Key generation logic for events is separate from a specific implementation within Redis/RedisHLL

Proposed Solution

See #411264 (comment 1392077260) for a further discussion and links to Proof-of-Concept MRs.