HTTP/3: proper uni stream closure detection. Previously, closure detection for server-initiated uni streams was not properly implemented. Instead, HTTP/3 code relied on QUIC code posting the read event and setting rev->error when it needed to close the stream. Then, regular uni stream read handler called c->recv() and received error, which closed the stream. This was an ad-hoc solution. If, for whatever reason, the read handler was called earlier, c->recv() would return 0, which would also close the stream. Now server-initiated uni streams have a separate read event handler for tracking stream closure. The handler calls c->recv(), which normally returns 0, but may return error in case of closure.