prometheus-node-exporter-lua: use uhttpd-mod-lua

prometheus-node-exporter-lua: use uhttpd-mod-lua

listen_ipv6 config option is removed and we now
listen on both ipv4 and ipv6 addresses.
HTTP keepalive is enabled and set to 70s by default.

With uhttpd-mod-lua there is a small change in behavior,
all code is loaded/parsed/executed once on startup as before,
but now each request is executed in his own fork, so we can't
keep a state between requests.

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>