Introduce forwarding log (journal) retention policy

[m-wilczynski]

When #3 is completed, it would be useful to have ability to automatically clean up forwarding log with entries older than X days (hours?).
Provided we have forward from outbox per 3 seconds, it makes (60/3)x60x24 = 28,800/day produced by OutboxForwarder.
If not introduced, log would get quite fat pretty quickly.