Stats tracker is easily cached, thus breaking jetpack_honor_dnt_header_for_stats

It would appear that the stats tracker is easily cached, thus breaking the purpose of the `jetpack_honor_dnt_header_for_stats` filter.

For example, if I were to clear Varnish and visit the site's main page and the most recent post from a browser *without* DNT set, the stats code would appear in the footer for all future visits (regardless of DNT setting) to the site's main page and the most recent post.

The opposite is also true, if I were to clear Varnish and visit the site's main page and the most recent post from a browser *with* DNT set, the stats code would not be added to the footer for all future visits (regardless of DNT setting) to the site's main page and the most recent post.

The breakdown appears to be that this is operating similar to PHP sessions, which means that the only way to effectively target DNT is to break the cache on every page load, which is not good.

Start by reproducing the issue with Varnish using browsers with and without DNT, then locate the stats tracker and the jetpack_honor_dnt_header_for_stats filter in the repository. Done means the stats code is not incorrectly shared between cached responses for different DNT settings, without requiring every page load to bypass caching.

Written by the indexing model from the issue text.