database/backfill_state: change type of first_expected_ts to BIGINT

database/upgrades/49-backfill-state-timestamp-bigint.sql

@@ -0,0 +1,7 @@
+-- v49: Convert first_expected_ts to BIGINT so that we can use zero-values.
+
+-- only: sqlite
+UPDATE backfill_state SET first_expected_ts=unixepoch(first_expected_ts);
+
+-- only: postgres
+ALTER TABLE backfill_state ALTER COLUMN first_expected_ts TYPE BIGINT;