refactor: replace writer cache with Caffeine

[vovapraded]

Summary

• replace TemporalCache with an internal Caffeine-backed writer cache and writer-aware TTL cleanup
• serialize acquire and cleanup per key, retain busy writers, and retry failed closes
• stop cleanup before pool shutdown and remove the obsolete TemporalCache API
• add deterministic expiration, close-failure, concurrency, and long-commit coverage

Testing

• ./gradlew :flink-connector-ytsaurus:test (198 tests, 2 skipped)
• ./gradlew shadowJar

Closes #44