Consider implementing pull / batch write to optimize flushing for streams

For streaming calls that can produce a batch of messages as a unit consider adding an interface that allows for a single flush to propagate through the framer to the transport.

In the case of outbound flow-controlled streams it should be possible for a producer to generate a sequence of messages that produce a single flush in response to the onReady (and onPayload) callbacks where the sequence of writes terminates when isReady becomes false.