Standardize claim status verbs

[carolynvs]

• Use past tense for final states.
• Use present participle, *-ing, for temporary states.
• Unknown is magic, and not a verb. It defies all standards.

This is a follow-up to https://github.com/cnabio/cnab-spec/pull/340/files#r387812107

[carolynvs]

😂 All my knowns are unknown and that's the way I like it