Resolve invalid snapshot pruning

[zivkovicmilos]

Description

This PR resolves a pending problem with snapshot pruning, where all snapshots would get pruned out if they incorrectly met the criteria (the snapshot prune selection mechanism didn't take into account that at least one snapshot needs to be left in the snapshot list in-memory).

This problem is only present in PoA networks.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Tested manually with a running cluster and small epoch sizes. Additionally, test cases that cover these scenarios have been added.

Additional comments

Fixes EVM-32

[codecov]

Codecov Report

Merging #791 (dee248b) into develop (9f1ce9b) will increase coverage by 0.03%.
The diff coverage is 80.00%.

@@             Coverage Diff             @@
##           develop     #791      +/-   ##
===========================================
+ Coverage    52.41%   52.45%   +0.03%     
===========================================
  Files          132      132              
  Lines        17494    17520      +26     
===========================================
+ Hits          9170     9190      +20     
- Misses        7657     7661       +4     
- Partials       667      669       +2     

Impacted Files	Coverage Δ
validators/store/snapshot/types.go	94.67% <80.00%> (-2.12%)	⬇️

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