luci-base: switch to lucihttp based POST data processing

Use the liblucihttp provided multipart and x-www-urlencoded body parsers
and drop the old Lua parsing code.

The C based data parsers are way faster than their old Lua counterparts
while producing less string garbage and more correct results.

While refactoring the luci.http.protocol code, also drop unused functions
and dead code, heavily reducing the module size.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>