InitProducerId may be blocked if least loaded node is not ready to send

From the logs of a failing producer that shows InitProducerId timing out after request timeout, it looks like we don't poll while waiting for transactional producer to be initialized and FindCoordinator request cannot be sent. The producer configuration used one bootstrap server and `max.in.flight.requests.per.connection=1`. The failing sequence:

1. Producer sends MetadataRequest to least loaded node (bootstrap server)
2. Producer is ready to send InitProducerId, needs to find transaction coordinator
3. Producer creates FindCoordinator request, but the only node known is the bootstrap server. Producer cannot send to this node since there is already the Metadata request in flight and max.inflight is 1.
4. Producer waits without polling, so Metadata response is not processed. InitProducerId times out eventually.
    

We need to update the condition used to determine whether Sender should poll() to fix this issue.