Implement EIP-3076 minimal slashing protection, using a filesystem database

[nalepae]

What type of PR is this?
Feature

What does this PR do? Why is it needed?
This pull request implements EIP-3076 minimal slashing protection, using a filesystem database.

Which issues(s) does this PR fix?

Fixes #12475

Other notes for review
This PR is designed to be read commit by commit.

This PR implements the flag --enable-minimal-slashing-protection in the validator client.
If this flag is used, then the former BoltDB validator.db database is not used any more.
Instead, a pure filesystem, YAML database is used, using a EIP-3076 minimal slashing protection database.

The filesystem structure is the following:

.
├── configuration.yaml
└── slashing-protection
    ├── 0x80f408a41341416f79e3be605c5f1b960169bea5753b9b8181f7466b4fa0b13addb250c9d068af263f820c113536f9f7.yaml
    ├── 0x84d7fa31925c559ef85531687b190d9299036625a73370153502bf1c086ac7d7e3006c15210c6f9bd4b518c1900e0f45.yaml
    └── 0xa17524bcf06d4fbf042e17dae68637368607f4fc9c28c104f49b12d1e0bed93fa3714f35ff05f4e518f789319c9b8f86.yaml

configuration.yaml contains data relative to:

• genesis validator root,
• proposer settings, and
• graffiti

An example of configuration.yaml file is:

genesisValidatorsRoot: 0x043db0d9a83813551ee2f33450d23797757d430911a9320530ad8a0eabc43efb
proposerSettings:
    proposerconfig:
        0x80f408a41341416f79e3be605c5f1b960169bea5753b9b8181f7466b4fa0b13addb250c9d068af263f820c113536f9f7:
            feerecipient: 0xef01d47ed04b181a2715D4050208f7d6e381bF42
            builder:
                enabled: true
                gaslimit: 30000000
                relays: []
        0xa17524bcf06d4fbf042e17dae68637368607f4fc9c28c104f49b12d1e0bed93fa3714f35ff05f4e518f789319c9b8f86:
            feerecipient: 0x801f87A1e14398e2ec9031CaBd585A84D92BC606
            builder:
                enabled: true
                gaslimit: 30000000
                relays: []
    defaultconfig:
        feerecipient: 0x1268AD189526AC0b386faF06eFfC46779c340eE6
        builder:
            enabled: true
            gaslimit: 30000000
            relays: []
graffiti:
    orderedindex: 0
    filehash: "0x0000000000000000000000000000000000000000000000000000000000000000"

In the slashing-protection directory, each file contains slashing protection data relative a given public key.
The public key itself is the name of the file.

An example of a file in the slashing-protection directory is:

latestSignedBlockSlot: 224123
lastSignedAttestationSourceEpoch: 224990
lastSignedAttestationTargetEpoch: 224991

The content of each field is self-describing.

Regarding anti-slashing strategy:

• For block proposals: The signature of an incoming block proposal will be refused if the slot of the incoming block is lower or equal to the latestSignedBlockSlot stored in the database.
• For attestations: The signature of an incoming attestation will be refused if:
  • the source epoch of the incoming attestation is lower to the lastSignedAttestationSourceEpoch stored in the database, or if
  • the target epoch of the incoming attestation is lower or equal to the lastSignedAttestationSourceEpoch stored in the database.

Strategy at VC start:

• If both minimal, filesystem AND complete, BoltDB databases are found, the VC refuses to start
• If --enable-minimal-slashing-protection is set AND a complete, BoltDB database is found, then a warning message is displayed indicating to the user how to convert the DB, and the VC continues with the complete, BoltDB database.
• If --enable-minimal-slashing-protection is NOT set AND a minimal, filesystem database is found, then the VC automatically convert the minimal, filesystem database into a complete, BoltDB one. (No data loss in this way)

To convert a complete database into a minimal one, the user can do:

validator db convert-complete-to-minimal [--datadir <datadir>]

😱 This PR has 10k+ new lines! 😱
Actualy, a lot of modified lines in this PR result in tests transformed from something like:

func test(t *testing.T) {
   ... := setup(t)
   // Content of the test
}

to:

func test(t *testing.T) {
   for _, isSlashingProtectionMinimal := range [...]bool{false, true} {
      ... := setup(t, isSlashingProtectionMinimal)
      // Content of the test
}

Git will consider all wrapped lines as new lines, even if only a few spaces had been added on the left.
For this reason, it is strongly advised to check the Hide whitespace option during the code review:

Benchmarks:
We create 2000 public keys, and we call, in parallel, SaveAttestationForPubKey on both complete, BoltDB and minimal, filesystem DBs.

Benchmark results:

• complete, BoltDB: ~130ms
• minimal, filesystem: ~127ms

Comments:
For complete, BoltDB database, SaveAttestationForPubKey fills a queue which is dumped into the DB every 100 ms (or if the queue, containing up to 2048 items, is full). It explains why the time needed is >= 100ms. If we decide to save only 10 attestations instead of 2000, the benchmark result is still ~104ms. Obviously, it cannot go under 100ms.