Important backpressure fix

6b7f354 fixes an elusive data loss bug. The bug can cause dropped outgoing chunks or messages under certain backpressure & corking scenarios.

We found the bug while working on HTTP chunked transfer-encoding streaming (see examples/VideoStreamerChunked.js) thanks to @GetThatCookie and others from dimdenGD/ultimate-express.

Impact

The bug has most likely manifested itself as a forceful connection closure due to client side protocol error. There should be no security implication (the bug does not leak data across sockets or cause overflows or crashes). A high traffic service with this fix will most likely see fewer reconnections due to client side protocol errors - improving overall service stability.