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.