Storing which messages have already been seen

[DanielHeath]

Could the RSS bee (and potentially others) have a way to keep track of which messages they have already seen (to avoid taking downstream actions multiple times)?

It looks like currently when you restart beehive, every RSS message is processed as if it were new.

[xeoncross]

I was just looking at this as well. Seems throwing a an embedded go store like boltdb/bbolt/badger onto beehive as a way to persist information would keep things simple yet provide this restart-safe durability.

[xeoncross]

Related to #26