import: Speed up create_subscription_events().
import: Speed up create_subscription_events().

The code was needlessly querying the DB to get full
objects for entities where we only needed user_id,
realm_id, and stream_id.

With my test data of ~1000 records this sped up the
function from ~8s to ~0.5s.  The speedup would probably
be even more for larger data sets.

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