Addresses Issue #8.
In its present form, restmail accumulates emails indefinitely. This can lead to the redis db overflowing memory.
This PR adds auto-expiration of all data after a configurable interval.
expire data after configurable interval
@jedp This looks good. The redis docs claim that redis scans 1000 randomly-selected keys per second, deleting the expired ones--given a reasonable TTL and a reasonably small number of emails received per minute, this should help a lot with minimizing memory usage. 🍻