Add indexes on dag_id column in referencing tables to speed up deletion of dag records

[pankajkoti]

When the dag records count gets huge, and users try to delete
DAG and DAG runs that are no longer needed or are stale, it
is observed that the deletion is significantly slow. The reason
for this is that the CASCADING DELETES are slow. Although,
we have foreign key constraints in the referencing tables, they
do not create an index implicitly on those columns (dag_id in
the referencing tables in this case). Hence, we're creating indexes
on the 5 of the 6 referencing tables for CASCADE DELETES to
speed up the deletion of records. In this PR, we're skipping to
add the index on the 6th table dag_owner_attributes as we're
facing a failure in the CI to find the constraint dag.dag_id for
that table. I plan to follow-up on the remaining 6th table it in a
separate PR. Without these indexes, it was observed that it takes
many hours to delete those records and it reduced to a few seconds
after adding those indexes.

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[ephraimbuddy]

LGTM