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.

vlib/strings/builder_js.v

@@ -9,11 +9,13 @@ mut:
 	buf []byte
 pub:
 	len int
+	initial_size int = 1
 }
 
 pub fn new_builder(initial_size int) Builder {
 	return Builder {
 		buf: make(0, initial_size, sizeof(byte))
+		initial_size: initial_size
 	}
 }
 
@@ -44,4 +46,6 @@ pub fn (b mut Builder) cut(n int) {
 }
 
 pub fn (b mut Builder) free() {
+	b.buf = make(0, b.initial_size, 1)
+	b.len = 0
 }