[performance] Add (optional) in-memory read cache

[hartmut-co-uk]

Description

For event processing logic where you might read specific, or the most recent data often, an in-memory caching layer would be beneficial to improve the performance, lower the network IO, take load off the database.

Outline

• optional, opt-in + config via builder
• fully configurable (size-based eviction vs. time-based expiration, eviction policy, ...)
• flag to enable/disable adding writes directly to the cache (vs. lazy load on read access)
• (allow to enable/config metrics, optional)

Implementation

• Proposed caching library to use: https://github.com/ben-manes/caffeine
• To consider: impl. with JSR-107 JCache to allow to abstract & choose the caching lib to use
• build in vs. using a wrapper
• bundled in vs. separate library (~ CachingReadOnlyKeyValueStore wrapper ??)

Tasks

• design
• implementation
• add unit tests
• write documentation
• create follow-up tasks