refactor(core-blockchain): reset missedBlocks before await call

[air1one]

Summary

Reset missedBlocks before await call in checkMissingBlocks.

Reason for that is the await call takes a bit of time, during which checkMissingBlocks can be called various times. If missedBlocks is not reset before it, we can end up calling this await checkNetworkHealth() a lot of times for nothing.

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

Merging #3598 into 2.6 will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              2.6    #3598   +/-   ##
=======================================
  Coverage   66.06%   66.07%           
=======================================
  Files         441      441           
  Lines       12506    12506           
  Branches     1716     1716           
=======================================
+ Hits         8262     8263    +1     
+ Misses       4211     4210    -1     
  Partials       33       33           

Impacted Files	Coverage Δ
packages/core-blockchain/src/blockchain.ts	66.03% <100.00%> (ø)
packages/core-transaction-pool/src/storage.ts	100.00% <0.00%> (+2.50%)	⬆️

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