filer: add write batching for FoundationDB store to improve throughput (#7708)

This addresses issue #7699 where FoundationDB filer store had low throughput
(~400-500 obj/s) due to each write operation creating a separate transaction.

Changes:
- Add writeBatcher that collects multiple writes into batched transactions
- New config options: batch_size (default: 100), batch_interval (default: 5ms)
- Batching provides ~5.7x throughput improvement (from ~456 to ~2600 obj/s)

Benchmark results with different batch sizes:
- batch_size=1:    ~456 obj/s (baseline, no batching)
- batch_size=10:   ~2621 obj/s (5.7x improvement)
- batch_size=16:   ~2514 obj/s (5.5x improvement)
- batch_size=100:  ~2617 obj/s (5.7x improvement)
- batch_size=1000: ~2593 obj/s (5.7x improvement)

The batch_interval timer (5ms) ensures writes are flushed promptly even
when batch is not full, providing good latency characteristics.

Addressed review feedback:
- Changed wait=false to wait=true in UpdateEntry/DeleteEntry to properly
  propagate errors to callers
- Fixed timer reset race condition by stopping and draining before reset

Fixes #7699

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