QUIC: avoid excessive buffer allocations in stream output. Previously, when a few bytes were send to a QUIC stream by the application, a 4K buffer was allocated for these bytes. Then a STREAM frame was created and that entire buffer was used as data for that frame. The frame with the buffer were in use up until the frame was acked by client. Meanwhile, when more bytes were send to the stream, more buffers were allocated and assigned as data to newer STREAM frames. In this scenario most buffer memory is unused. Now the unused part of the stream output buffer is available for further stream output while earlier parts of the buffer are waiting to be acked. This is achieved by splitting the output buffer.

--- a/src/event/quic/ngx_event_quic.h
+++ b/src/event/quic/ngx_event_quic.h
@@ -63,6 +63,7 @@ struct ngx_quic_stream_s {
     uint64_t                   recv_last;
     uint64_t                   final_size;
     ngx_chain_t               *in;
+    ngx_chain_t               *out;
     ngx_uint_t                 cancelable;  /* unsigned  cancelable:1; */
 };