Use ref_src_workflow_id to find workflow ID, add index
Add a temporary index to make lookups by ref_src_workflow_id and
ref_src_wiki efficient.
The phases have been shifted.
Phase 2 - Just temporary indices
Phase 3 - Actual backfill
Phase 4 - Final schema changes
Bug: T111084
Change-Id: I695d48e56f54303f8694b090569cd2b62322fc3b