[HackerOne-2452182] Store `Proposal` and `SignedProposals` to disk and remove proposal expiration

[raychu86]

Motivation

This PR has two changes to address https://github.com/AleoHQ/snarkOS/issues/3171:

1. Reverted proposal expiration introduced in https://github.com/AleoHQ/snarkOS/pull/3202.

   • This approach was unsafe because it violated the non-equivocation properties of our BFT protocol.
   • A community member @ghostant-1017 also highlighted this issue in a bug bounty finding [HackerOne-2452182].

2. The ProposalCache (pending Proposal and SignedProposals) will be stored to a file on shutdown, loaded on bootup, and cleared with snarkos clean.

   • Ensures that honest validators do not create multiple proposals on the same round
   • Ensures that we do not sign additional validator proposals on the same round after a reboot.

Implications

• If enough (a majority) of validators reboot their nodes at the same time, there could be a halting case.

  • If we signed proposals past our ledger round, but all nodes have thrown away their Storage, then we won't have the signatures required to reconstruct the original proposal state.
  • This will need to be remedied by having all validators delete their proposal_cache file and reboot.

• Validators looking to swap machines will need to migrate their proposal_cache file between machines to ensure honest behavior.

Test Plan

Extensive local and burn-in testing will need to be performed.

Related PRs

Reverts: https://github.com/AleoHQ/snarkOS/pull/3202. An extension of: https://github.com/AleoHQ/snarkOS/pull/3200

TODO

• [ ] Add unit tests for serialization
• [ ] Add unit tests to ensure the enforcement properties with the new stored proposal.
• [ ] Consider adding new log messages to inform validators of the importance of the storage.