Two things:
- We could run into a write error, which would block the progress
forever without an error. This because the writer routine exited, while
the reader was just blocked on sending to it.
- After a failed migration, inserts could fail with unique index
constraint errors because we are reusing the sequence numbers from the
original database. Add a drop folder to the start of migration to handle
this.
Additionally, the drop folder will clear out broken database files due
to killed migrations.