Pruning - Clean up objects after pruning epoch

[jonastheis]

How it works

For each pruning epoch, creates new storage (bucket) for incoming data. When we have enough buckets, the old ones can be removed.

Discussions

• Which timestamp to use, issuance timestamap or received timestamp?
• If a node is down, get unsynced for 5, 6 hours, how should we handle data from the gap to sync?