causal-01-ordering: refine some wording
- "transitive reduction" is too technical for the opening paragraph
- freshness is a more advanced property than just replay protection and isn't achieved by the things so far described
more on partial and causal ordering
- add notation for |perp|, "causally independent"
- explain the principle of eliminating redundant information
- explain freshness consistency, and show that transitive reduction entails it
- summarise invariants in formal language and suggest tests