Mweb 1017 caching

@nandy-andy @aquilax @hakubo @rogatty It is an attempt to provide control over caching our responses:

1) Caching all the GET responses from the API
2) Re-enabling MD5 ETags provided by Express (render/ overrides them and sets the ETag as a mix of last update timestamp and cachebuster)
3) tile_set/ and map/ use surrogate keys for purging as they have querystring params we cannot control
What names? I think `cachingUtils` are fine :smile_cat: also [the caching times "constants" names](https://github.com/Wikia/interactive-maps/blob/MWEB-1017-caching/lib/cachingUtils.js#L9) are consistent with [names of the same stuff in our MW application](https://github.com/Wikia/app/blob/a86ae349828b1120905d1e3e74ffb925d6e736d3/includes/wikia/nirvana/WikiaResponse.class.php#L43).