HttpResponse: write the status, Date and Content-Length lines in one piece each

[nigrosimone]

A response head is about ten AsyncSocket::write calls, and each one repeats the closed check, the loop data lookup and the cork checks. This writes the status line, the Content-Length line and the uWebSockets mark in one piece each, and the Date line from a copy LoopData prepares when the date ticks: from ten writes to four for a minimal response.

Bytes are identical (static, JS handler, chunked, status over 64 chars, 404, 70 KB body, WebSocket upgrade, with and without the mark).

User-space instructions per request, two copies of the build each: arm64 (OCI A1, Node 26) -18% on a DeclarativeResponse, -4% on a JS handler with status, two headers and end; x64 (Node 24) -20% and -5%. Requests per second swing ±20% between rounds of the same build on both boxes, so a gain this size does not show there.