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

go.mod

@@ -123,7 +123,7 @@ require (
 	github.com/Jille/raft-grpc-transport v1.6.1
 	github.com/ThreeDotsLabs/watermill v1.5.1
 	github.com/a-h/templ v0.3.943
-	github.com/apple/foundationdb/bindings/go v0.0.0-20240515141816-262c6fe778ad
+	github.com/apple/foundationdb/bindings/go v0.0.0-20250911184653-27f7192f47c3
 	github.com/arangodb/go-driver v1.6.9
 	github.com/armon/go-metrics v0.4.1
 	github.com/aws/aws-sdk-go-v2 v1.40.1