Implement Go's pcd (per-client deferred flush) pattern to reduce write-loop
wakeups during fan-out delivery, optimize ack reply string construction with
stack-based formatting, cache CompiledFilter on ConsumerHandle, and pool
fetch message lists. Durable consumer fetch improves from 0.60x to 0.74x Go.
Implement Go-parity background flush loop (coalesce 16KB/8ms) in MsgBlock/FileStore,
replace O(n) GetStateAsync with incremental counters, skip PruneExpired/LoadAsync/
PrunePerSubject when not needed, and bypass RAFT for single-replica streams. Fix counter
tracking bugs in RemoveMsg/EraseMsg/TTL expiry and ObjectDisposedException races in
flush loop disposal. FileStore optimizations verified with 3112/3112 JetStream tests
passing; async publish benchmark remains at ~174 msg/s due to E2E protocol path bottleneck.
Add ResetToSequence to ConsumerManager that updates NextSequence,
clears AckProcessor state via new ClearAll(), and zeroes PendingBytes.
Add AckProcessor.SetAckFloor() that prunes pending entries below the
new floor. Go reference: consumer.go:4241 processResetReq.
Heartbeat frames now include Nats-Pending-Messages and Nats-Pending-Bytes
headers populated from the ConsumerHandle. Flow control frames increment
FlowControlPendingCount; AcknowledgeFlowControl() decrements it. IsFlowControlStalled
returns true when pending count reaches MaxFlowControlPending (2).
Go reference: consumer.go:5222 (sendIdleHeartbeat), consumer.go:5495 (sendFlowControl).
Adds PauseUntilUtc to ConsumerHandle, a new Pause(DateTime) overload,
Resume, IsPaused, and GetPauseUntil to ConsumerManager. A System.Threading.Timer
fires when the deadline passes and calls AutoResume, raising OnAutoResumed so
tests can synchronise via SemaphoreSlim instead of Task.Delay. ConsumerManager
now implements IDisposable to clean up outstanding timers. Timer is also
cancelled on explicit Resume and Delete.
Go reference: consumer.go (pauseConsumer / resumeConsumer / isPaused).
