Consider implementing pull / batch write to optimize flushing for streams

[louiscryan]

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.

[ejona86]

For context, the main goal of this support is to reduce the number of buffers we use to write such messages. At present, each message would be in its own buffer; if each message is small we will waste a lot of memory.