wireproto: buffer output frames when in half duplex mode Previously, when told that a response was ready, the server reactor would instruct the caller to send frames immediately. This was OK as an initial implementation. But it would not work for half-duplex connections where the sender can't receive until all data has been transmitted - such as httplib based clients. In this commit, we teach the reactor that output frames should be buffered until end of input is seen. This required a new event to inform the reactor of end of input. The result from that event will instruct the consumer to send all buffered frames. The HTTP server is buffered by default. This change effectively hides the complexity of buffering within the reactor so that transports need not be concerned about it. This helps keep the transports "dumb" and will make implementing multiple requests-responses per atomic exchange (like an HTTP request) much simpler. Differential Revision: https://phab.mercurial-scm.org/D2860