Add method to use buffered IO (mod-lua)

[champtar]

Right now uhttpd.send() just calls write() to stdout, thus potentially doing lots of small writes.
Using buffered io for prometheus-node-exporter-lua reduces the netclass collector scrape time from 230ms to 170ms
https://github.com/openwrt/packages/pull/26860/files#diff-bfae82b02aa972d6c7a6848c12c61fe5af113318ddaacde9db8d89692112fe4bR90

Is there a reason not to use buffered io I'm missing ?
Should uhttpd.send() use buffered io by default ?