IPNS garbage collection

As far as I can see in all IPNS entries get stored in the MapDatastore of the DHT, but old entries won't be removed once they reached the end of life. This could lead to a lot of memory usage and potentially crash the node.

In the DHT Datastore the max record age is 36 Hours, but are only removed on trying to receive the entry. If an old entry is not asked by any node it will not be removed, this can lead to a buildup of a lot of entries.

I propose that all IPNS entries should be garbage collected in a regular interval and removed if their end of life is reached, which can be way earlier than the MaxRecordTime of the DHT.