[Task] Use Milestones for ordering DID Messages

[JelleMillenaar]

Description

As described in the iota DID Method Specification, message ordering happens if two or more messages try to take the same place in the state machine. The ordering is meant to make use of the milestones of the IOTA network, but this is not yet implemented. In addition, we received a report that ordering based on the hash (after milestones), wasn't done according to how it is described in the method spec, that needs to be checked.

Motivation

Have deterministic ordering that cannot be changed after a DID message has been confirmed in the network.

To-do list

Create a task-specific to-do list . Please link PRs that match the To-do list item behind the item after it has been submitted.

• Add milestone checking to the ordering mechanism
• Check hashing based ordering to match the spec