Option to clean up the data file

[gfa]

Hello

this is a whistlist bug reported to Debian which I'm forwarding

My data file regularly grows to hundreds of megabytes, until I run

 r2e reset && r2e run --no-send

but there's a race condition in that.

Wouldn't it be possible to run e.g.

r2e run --forget-old

such that the data file gets rewritten with just the IDs that are
served by the feed? Since RSS feeds usually operate like a queue,
and older elements are unlikely to show up again, this would help
reduce the size of the data file without the race condition.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=896516

[Ekleog]

This is a duplicate of #49