fix(core-blockchain): reset last downloaded block

[spkjp]

Proposed changes

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build (changes that affect the build system)
• Docs (documentation only changes)
• Test (adding missing tests or fixing existing tests)
• Other... Please describe:

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov-io]

Codecov Report

Merging #1692 into develop will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #1692      +/-   ##
===========================================
- Coverage    74.96%   74.95%   -0.01%     
===========================================
  Files          444      444              
  Lines         7962     7963       +1     
  Branches      1048     1048              
===========================================
  Hits          5969     5969              
- Misses        1770     1771       +1     
  Partials       223      223

Impacted Files	Coverage Δ
packages/core-blockchain/lib/blockchain.js	75.47% <0%> (-0.36%)	⬇️

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