Deduplicate concurrent in-flight select() requests in TTLSelectCachingDAO

[abdelaziz-mahdy]

Problem

TTLSelectCachingDAO only caches resolved results. When multiple concurrent callers issue identical select() queries before the first one resolves, each caller fires a separate server request. In batch scenarios (e.g., 300 identical queries triggered simultaneously), this produces 300 redundant network round-trips.

Solution

Store the in-flight promise in the cache immediately on miss. Concurrent callers with the same cache key receive the shared promise instead of starting new requests. When the promise resolves, replace it with the cloned result for subsequent cache hits. On error, remove the failed entry so the next caller retries.

Changes

• Store the delegate's promise in this.cache[key] before it resolves, so concurrent select_() calls with the same key share one request
• Clone the result when returning from an in-flight promise hit, preserving the existing cache corruption protection
• Remove failed entries on error instead of clearing the entire cache