Make some stats more efficient

[pkevan]

When we call update_stats, we repeat the call, causing 5 updates, rather than just one (since the option is the same):

https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/plugins/camptix/camptix.php#L2697-L2703

We could create a new method which allows multiple stats to be update at the same time, to avoid options being overwritten, if there happens to be several updates at the same time.