Enable tx prefetcher for multiple blocks

[kjhman21]

Proposed changes

• Previously, the tx prefetcher was enabled when only one block is passed into BlockChain.insertChain().
• After sync test, we found that the tx prefetcher is useful when multiple blocks are passed to that function.
• Hence, we enable tx prefetcher if multiple blocks are passed.

Types of changes

Please put an x in the boxes related to your change.

• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

• Fix the prefech logic for multiple txs of the same sender in a block #911

[ehnuje]

Please change or remove the comment accordingly 😅

[kjhman21]

Updated. Please take a look. :)