PoS validator deactivation + reactivation

[tzemanovic]

add transactions and update PoS validity predicate for validator deactivation + reactivation

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[cwgoes]

@brentstone do you know the status of this?

[brentstone]

In general, not fully implemented yet and still relevant. I've started to address this in #892, as cubic slashing requires jailing / deactivating a validator. Could also have it in a separate PR if that is preferable.