Record usage info from ParserOutput to the database.

Note: this does not yet filter updates by aspect used,
see I9ce6a2e9 for that.

DEPLOYMENT: the new wbc_entity_usage table has to be populated for
ChangeHandler to detect which pages use a given entity (pending
bug 71050). Until then, keep useLegacyUsageIndex set to true,
so usage tracking is done based on the wb_items_per_site table.

Bug: 64589
Change-Id: I674915557397a827f250261c7d11afc71f8f646f
Merge "Record usage info from ParserOutput to the database."