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).
