When Persistence is enabled, allow old data to expire

[jonesnc]

Is your feature request related to a problem? Please describe.
I want to prevent the flower db file from growing too large (>100MB, for example).

Describe the solution you'd like
I want to be able to set a config var like PERSISTENT_EXPIRATION_TIMEDELTA for example to 30 days,let's say, and any data that is stored in the Flower db file that is older than PERSISTENT_EXPIRATION_TIMEDELTA would be "flushed" / "deleted" from the flower db file.

If this is already possible, please let me know, as I couldn't find this functionality in the documentation. Thanks!

[max-wittig]

See: #1310