Use pparam major version change to trigger hard-fork

[agaffney]

We're currently triggering hard-forks based on the era of incoming blocks, but we now have protocol param tracking available. Use a change in the protocol major version in the (active) pparams to trigger the appropriate era's HF mechanism. Some eras have multiple major protocol versions, so we'll need a mapping based on the protocol version.

Useful links:

https://cardano-course.gitbook.io/cardano-course/handbook/cardano-governance/update-proposals
https://cardano.org/hardforks/