[Spec] simulation assumptions

[nrryuya]

• Verifiability of messages
  • Do not simulate attacks by invalid messages
  • Assume validators can verify messages (See [Spec] How to check whether some data is valid message or valid state efficiently? cbc-beacon-chain#1 (comment))
• The total order of messages from the same sender
  • Do not reorder messages from the same sender
  • Assume this is solved by P2P layer (e.g. "pending" rule)
    • DoS attack vector (See [Spec] "Pending" messages and DDoS attack cbc-beacon-chain#8)
  • Similarly, in "vote-by-block" consensus, we can assume the total order of all messages