Cleanup old checkpoints from finalization state

[frolosofsky]

This fix significantly reduces individual finalization state size.

In a test, I run 1 proposer with 4 finalizers with configuration epoch_length=2.
After processing ~9k blocks and ~4.5k epochs, finalization state size was 767599.
After applying this PR, it has been reduced to 196002. More finalizers we have,
more memory we save, because checkpoints map holds votes from every finalizer.

Relates to #1073.

[scravy]

The lint step failed in a weird way:

I have a feeling this has something to do with the scripted diff merged in #1072 as the messages refers to a52d88e

@cornelius Can you have a look?

[scravy]

Concept ACK 0bc5e57

[kostyantyn]

ConceptACK 0bc5e57

I see it as a great quick win! This checkpoint design grows n^2, and with the low effort, we did it linearly. I have in mind to completely delete checkpoints as I don't see a need for them if we can look back for the previous finalization state, but that requires a significantly larger change/refactoring, and after this PR, it won't give us a drastic improvement so it can wait.

[Gnappuraz]

I'm looking at finalizationstate.cpp:924
GetCheckpoint(vote.m_target_epoch).m_vote_set.insert(vote.m_validator_address);
and checking if actually we always pass by the ValidateVote function, if not, is an easy way to crash the node with an old vote.

As a bare minimum I think we should change anyway the log messages in finalizationstate.cpp:322 "%s: target_epoch=%d is in the future.\n", __func__, and following since is not necessarily an epoch in the future anymore.

[cornelius]

The weird lint failure should be fixed by #1084 now.

[Ruteri]

Tested 0bc5e57 and while it does improve overall memory consumption, it does not resolve the syncing issue #1073.

[frolosofsky]

I'm looking at finalizationstate.cpp:924
GetCheckpoint(vote.m_target_epoch).m_vote_set.insert(vote.m_validator_address);
and check if actually we always pass by the ValidateVote function, if not, is an easy way to crash the node with an old vote.

As a bare minimum I think we should change anyway the log messages in finalizationstate.cpp:322 "%s: target_epoch=%d is in the future.\n", __func__, and following since is not necessarily an epoch in the future anymore.

Fixed/checked in 8fe4a24.

[Gnappuraz]

except my last comment, utACK d751b58

[Ruteri]

ConceptACK d751b58
Tested resource usage - significantly lowers memory usage.