[PERF] Concurrent compaction — run multiple compaction threads in parallel for different CFs

[ElioNeto]

Description

Currently ApexStore runs compaction in a single background thread (maybe_compact). Multiple column families compact sequentially. Competitive engines support concurrent compaction across column families.

Impact

• With 8+ column families, compaction becomes a bottleneck
• Write stalls while waiting for single-thread compaction to finish
• CPU cores are underutilized during compaction I/O

Proposed Implementation

1. Change compaction_thread from Mutex<Option<JoinHandle>> to Mutex<Vec<JoinHandle>>
2. Allow up to N concurrent compactions (configurable via compaction.max_concurrent)
3. Assign one compaction per column family
4. Use a semaphore to limit total concurrent compactions
5. Ensure close() waits for all compaction threads to finish

Labels

[ElioNeto]

Fixed in commit 1d8c830.