Only invalidate tiles which are affected by a update

[Baschdl]

If I understand the cache invalidation logic in

Windshaft-cartodb/lib/cache/layergroup-affected-tables.js

Lines 5 to 8 in a19e9a7

	function LayergroupAffectedTables () {
	// dbname + layergroupId -> affected tables cache
	this.cache = new LruCache({ max: 2000 });
	}

correctly, all tiles of a datasets are purged when updates to the dataset happen.
It would be very helpful for big datasets to only invalidate the tiles which are affected by a update.