Make backfill batch selection exclude rows inserted or updated after backfill start

[andrew-farries]

Backfill only rows present at backfill start. This is third approach to solving #583. The previous two are:

• Make backfill batch selection exclude rows inserted or updated after backfill start #634
• Make backfill batch selection exclude rows inserted or updated after backfill start #634

This is the most direct approach to solving the problem. At the same time as the up/down triggers are created to perform a backfill, a _pgroll_needs_backfill column is also created on the table to be backfilled. The column has a DEFAULT of true; the constant default ensures that this extra column can be added quickly without a lengthy ACCESS_EXCLUSIVE lock. The column is removed when the the operation is rolled back or completed.

The up/down triggers are modified to set _pgroll_needs_backfill to false whenever they update a row.

The backfill itself is updated to select only rows having _pgroll_needs_backfill set to true - this ensures that only rows created before the triggers were installed are updated by the backfill. The backfill process still needs to read every row in the table, including those inserted/updated after backfill start, but only those rows created before backfill start will be updated.

The main disadvantage of this approach is that backfill now requires an extra column to be created on the target table.

NOTE: We'd need to update some docs (especially the tutorial) to mention this new column if we go with this solution.