Implement changes from 1st protocol audit

[anorth]

See FIP PR filecoin-project/FIPs#985

Tasks

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1. Replace ECCompatibleChains with C (give a better name) which is updated when node observes a value that could have been decided by another participant
2. Remove the ECUpdate (ReceiveAcceptable) method
3. Implement might-have-been-decided predicate (replaced with strong-quorum-of-bottom check)
4. Require evidence for PREPARE when round > 0
5. Carefully check the message validation and justification methods match spec
6. Carefully check for other changes to the protocol pseudocode such as timeouts, thresholds etc
7. Implement #136