fix sqlite performance issue

[Gaudy Blanco (gaudyb)]

Description

Adds a backward-compatible Cache.get_many() API for efficient bulk cache reads.

Provides a default implementation for custom Cache implementations.
Adds direct implementations for MemoryCache and NoopCache.
Optimizes SQLiteCache to:
Use one thread dispatch and one connection per bulk read.
Deduplicate keys and omit missing entries.
Split large requests into bounded 500-key SQL batches.
Preserve namespace isolation.
Decode values consistently with get().
Safely remove malformed entries only when their payload has not changed.
Adds shared contract tests and SQLite-specific batching, concurrency, cleanup, connection reuse, and dispatch-count tests.
Adds an opt-in benchmark comparing Memory, JSON, and SQLite bulk reads.
Updates cache documentation and release notes.

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).