reorient backfill progress around partitions instead of runs
Previously, we were always keeping track of progress for backfills by tracking the
associated run count. This gets misleading when backfill runs get reexecuted. It gets
harder to keep track of overall status in these cases. For example, if I schedule a single
partition backfill job, but I re-execute it 5 times, the progress on that backfill job
will show 5/5 instead of 1/1. Previous bugs (that have since been fixed) for this type
of confusion involved displaying negative numbers, etc.
This diff changes the display status for a backfill job by grouping runs by partition, and then
showing status over partitions.
Test Plan: Created backfill, reexecuted a bunch of partitions.
Reviewers: sandyryza, dish
Reviewed By: sandyryza
Differential Revision: https://dagster.phacility.com/D7165