rename variable name to prevent collision with nginx 1.12 builtin var

[tykling]

nginx 1.12 does not start with this module built in. This is because the variable name conflicts with the new builtin $request_id variable in nginx: nginx/nginx@f315b7a

[tykling]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218956

[gabor]

thanks for the pull-request, but i think a better approach is to simply not use this module anymore, and simply use the new $request_id builtin. i see that the freebsd-bug was handled this way too.