http2: adjust stream buffer size

Adjust stream buffer size to allow full 4 default-sized frames
including their frame headers to allow more efficient sending
of data to the socket.

PR-URL: https://github.com/nodejs/node/pull/16445


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>