cassandra: Make content_missing query in batches
Instead of calling content_find() for each object, which needs to make two queries for each.
Given the latency of Cassandra queries, this should be a significant speed-up (possibly up to 100 times faster, as this is the value of PARTITION_KEY_RESTRICTION_MAX_SIZE).
This also changes the schema, because CQL does not allow doing IN
queries on compound partition keys.
Test Plan
Both branches are already covered by existing tests
Migrated from D6118 (view on Phabricator)