Add ocap revocation implementation and tests

[dlongley]

Add tests to ensure that operations invoked using ocaps that were revoked at the basisBlockHeight time of the event that the operation occurs in are disallowed.

[EDIT] changed basisHash to basisBlockHeight

[mattcollier]

One example of this would be making an update to an existing DID where the key used has been revoked/removed prior to the new update operation.

@dlongley what is the key revocation story on Veres One? Are keys removed from the capabilities array or are they somehow marked as revoked or some combination of both of these things?

[dlongley]

@mattcollier,

Currently, key revocation can be done by removing the key or adding a revoked property to the key with a user specified date as the value.