Virtual Milestones: Epochs and Commitments

[jonastheis]

Regular checkpoints into the network are key for agreeing on a finalized state, so data can be compressed into a local snapshot and pruned. These checkpoints must also provide a mechanism to prove inclusion after the Tangle has been pruned. Therefore, each message should contain a commitment to a time slice (epoch).
This epic describes the implementation for the Virtual Milestones proposal's epochs and commitments.

For now:

• Epochs manager: translates time to epoch number and viceversa. Should be able to return EC for latest commitable epoch. Reference old epoch implementation.
  • Multiple of TSC time.
• Notarization Manager will make use of the Epoch Manager and will create epoch commitments reference.
  • When to consider an epoch commitable.
  • Pending Branch Counter.
  • Orphanage detection:
    • remove stale pending objects from epochs to not starve epoch confirmation.
    • (to be discussed and studied) remove confirmed stuff that ended up in an epoch commitment. A part of the Tangle got orphaned and we have to remove its weight from its past cone: changing confirmations.
  • Subscribe to confirmation events for messages, transactions and branches: for mutation and tangle commitments.
  • Subscribe to transaction confirmation for state commitment.
  • EC model and serialization.
• Messagefactory call Notarization Manager to obtain the EC.

For later:

• State Manager wraps Epochs Manager and should be able to return a new Tangle object based on an old epoch state.
  • Should also be able to decide if we need to switch state to a heavier ECC.
  • Obtain and reset the state to the forking point.
  • Resync from forking point.
• Pruning capability: we will always have just a few database instances running (hundreds of epochs each or so). The API for the storage will mostly stay the same, a storage middleware will take care of reading / writing to the appropriate database instance.
• TSA won't select tips with different EC. If a different EC is detected the State Manager should be invoked.

Spec: https://hackmd.io/gYleE-ESRJGpmsNBDnMBxA?both (WIP)