MINOR: lua: move the http service context out of appctx.ctx

MINOR: lua: move the http service context out of appctx.ctx

Just like for the TCP service, let's move the context away from
appctx.ctx. A new struct hlua_http_ctx was defined, reserved in
hlua_applet_http_init() and used everywhere else. Similarly, the
task dump code will no more report decoded stack traces in case
these services would be involved. That may be solved later.