Implement more advanced anchor contracts

[OR13]

trafficstars

We have requests for supporting a more complex anchor contract. It would be nice to add support for staking and rate limiting by address.

As a stretch goal, we could show that a migration from the default anchor contract to a more restricted one is possible, while preserving DIDs. This would give us event more confidence to proceed with the simple and more open contract we have today, knowing that we can protect users later with a migration.