Add tmp index to members table

Related to #534094 (closed)

What does this MR do and why?

Adds a temporary index for a BBM. See !225317 (merged). Without it, the BBM would take ~3 months.

Database Review

Index creation on postgres.ai:

exec CREATE INDEX tmp_idx_members_on_access_level_source_type_requested_at_id ON members USING btree (access_level, source_type, requested_at, id);

Session: webui-i51716 — Duration: 4.494 min

Query plan: postgres.ai

explain SELECT "members"."id" FROM "members" WHERE "members"."access_level" = 5 AND "members"."source_type" = 'Namespace' AND "members"."requested_at" IS NULL ORDER BY "members"."id" ASC LIMIT 1000

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