Fix LastSeenMessage and Validators count issue

[ZhangTao1596]

Close #1799

Changes:

• Use dictionary to store LastSeenMessage in order to cover scenario like this:
  • Validators replaced by another account without changing validators count
  • Both validators and validators count changed
• Fix bug in method of counting the number of validators based on votes

How to test and verify the impact of these changes?

• Voting to change validators count or validators to check whether it works right
• Stop some consensus nodes to check whether failed count is right
• Don't start validator node you want to add in consensus to check whether failed count is right
• Switch old version to this fix version and resync blocks to check whether it works well

Where in the software does this update applies to?

• Consensus
• Persistence

@erikzhang @shargon How about fix like this?

[superboyiii]

Tested. PASS

Vote 4 CN to 7 CN, switch smoothly and no Index was outside the bounds of the array.

[shargon]

The code looks good but I see the following points:

• We save the lastSeen by publicKey instead of by index, but it's needed? this requires comparing by public key, which is slower than an array, but is more accurate.
• It does not contain a cleaning logic, if we change the CN each block will increase the memory of the node until break it.

[ZhangTao1596]

• It does not contain a cleaning logic, if we change the CN each block will increase the memory of the node until break it.

If consensus changed, LastSeenMesage will be re-new.

[shargon]

Why this change?

[ZhangTao1596]

Because we use Votes.Length - 1 as index when add votes.

neo/neo/Ledger/Blockchain.cs

Line 481 in 75df7cd

snapshot.ValidatorsCount.GetAndChange().Votes[account.Votes.Length - 1] += output.Value;

neo/neo/Ledger/Blockchain.cs

Line 509 in 75df7cd

snapshot.ValidatorsCount.GetAndChange().Votes[account.Votes.Length - 1] -= out_prev.Value;

And as @superboyiii described in #1799. Only 6 nodes became validators after vote 7 validators.

[ZhangTao1596]

@igormcoelho @vncoelho @erikzhang @Tommo-L Please have a look?

[superboyiii]

@erikzhang Shall we merge?

[superboyiii]

Retest it based on my local repository (merged this with neox-2.x and master-2.x)
Result: PASS
Test steps:

1. Create 7 nodes private net via 2.10.3, vote for 7 address (one is taked placed by a new address).
2. Sync the latest version and take place the old consensus.
3. Revote for these new consensus (vote for original candidates)
4. Ensure no error and block generated normally.

@shargon Please help us merge this.