tortoise: tortoise state loading will be growing in time

[dshulyak]

After persistent state was removed tortoise loads state from meshdb and atxdb when the node is started. In the initial implementation i am also re-verifying everything, as it allowed to reuse code for rerun almost as is.

However after #3000 is merged rerun in full mode will be rather slow. So instead of verifying when node is booted we can just load state.

the end goal is to make restart fast, we can achieve that:

• by eliminating bottlenecks in tortoise
• by snapshotting tortoise when node is stopped gracefully
• by loading state without verifications
• by offloading state loading into background, and blocking components that rely on tortoise

[dshulyak]

Note: we don't need state in full.weight for verified layers, but we need to know which ballots are good, because the definition is recursive.

[dshulyak]

Also need to compute real margins for layers between verified and processed, as we will use them for voting. So it is not completely possible to avoid vote counting. Will postpone this change.

[dshulyak]

We can do this after mutable ballots will be implemented. They allow us to verify goodness of the ballot without recursively decoding all previous votes, instead we can just compare if ballot encoded opinion is identical to local opinion.

[countvonzero]

minor issue i encountered is that the loading is not interrupt-able with sigterm as it doesn't check context cancel.