vweb: continue after bad http client connection; performance fixes

* Enable compiling vweb with -prod (by supressing 'declared and not used' warning about 'reset') .

* Fix http responses (now wrk is happy and shows no errors) by adding a Content-Length header.

* Fix -g compilation for urllib.v .

* vweb: println action= only in debug mode.

* vweb: max request headers counting fix.

* Make vweb.html get a 'ctx mut Context' param, just like the other methods.

* vweb: simplify add_header.

* Use a string builder for the most common html case so that the response http text can be send in one go.

* vweb: reduce _STR/string interpolation usage in the most common html response case.

* vweb: refactor common http response formatting into Context.send_response_to_client/2 method.

examples/vweb/vweb_example.v

@@ -22,7 +22,7 @@ pub fn (app mut App) init() {
 	app.vweb.handle_static('.')
 }
 
-pub fn (app & App) json_endpoint() {
+pub fn (app mut App) json_endpoint() {
 	app.vweb.json('{"a": 3}')
 }