feat: cli,events: speed up backfill with temporary index

[rvagg]

From experience running this:

1. Add a warning about running this against a node that's actively collecting events, you'll have trouble
2. Use IMMEDIATE transactions so we know as soon as we start a transaction whether we can get an exclusive lock for a write or not so we can error more easily.
3. Add an optional temporary index for our query which significantly speeds up this process (a lot).
4. Run an optional vacuum after we're done.