When tracking label usage, we record which language is used, so we only purge the page when that language changes.
Similarly, we should record which statement group is used (by property ID), so we only purge the page when that statement group changes.
We will enable this on `elwiki` initially in order to evaluate the number of new usage tracked.
Implementation issues:
* We will write a lot (?) more rows to wbc_entity_usage. This will be evaluated based on the data from elwiki
* We currently pass the entire serialized Entity to Lua. We'd have to use magic accessors to track usage of individual bits of the structure.
** We could however easily track access via a specialized Lua function for getting or formatting Statement groups.
#patch-for-review:
[x] https://gerrit.wikimedia.org/r/336542
[x] https://gerrit.wikimedia.org/r/353154
[x] https://gerrit.wikimedia.org/r/355101
[x] https://gerrit.wikimedia.org/r/355104
[x] https://gerrit.wikimedia.org/r/375544
[x] https://gerrit.wikimedia.org/r/382414
[x] https://gerrit.wikimedia.org/r/383439
[x] https://gerrit.wikimedia.org/r/383440
[x] https://gerrit.wikimedia.org/r/383556
[] https://gerrit.wikimedia.org/r/384592