0416 ran DROP INDEX CONCURRENTLY at boot, which waits without bound for
every transaction on the table; a pg_dump or a backfill outlasted the
start period and the next boot stopped at P3009. The migration now tries
a plain drop with a two-second lock wait and tolerates the timeout, and
the nightly measurement maintenance drops what is left concurrently under
a bounded wait, retrying the next night. A fresh or shadow database takes
the lock at once, so the history still ends where the schema does. The
guard refuses a concurrent drop in a migration and ties every retired
index to a tolerant one.
