Do not use ngx.var inside mongol's code

[hololoev]

Hey guys!

There is a string: https://github.com/bigplum/lua-resty-mongol/blob/master/lib/resty/mongol/object_id.lua#L56

ngx.var api is disabled inside ssl_certificate_by_lua* derectives. Is it really necessary to use ngx.var? Mb we can use something instead?

[kedare]

You can replace the value by a fixed value in the worst case, it should work on most of the case (In my case I'm only reading so I don't care of the ObjectID generation)