istanbul: Fail fast VerifyHeaders, Fix handlerWg

[blukat29]

Proposed changes

This PR contains three small fixes.

• Fix VerifyHeaders to fail fast when one of the headers is errored.
  • Consensys/quorum@35f1c3f (#1042)
  • When the VerifyHeaders() and PreprocessHeaderVerification() iterates through a list of consecutive headers, when it encounters a bad header then the rest of them are skipped.
  • Note that in Klaytn VerifyHeaders() is disabled because CanVerifyHeadersConcurrently() is false. So this fix should have no effect.
• Fix handlerWg location with respect to the standard use case.
  • Consensys/quorum@1b7c47f (#1248)
  • When waiting for background goroutines, it is intuitive to follow this wg.Add(1) -> go loop() -> wg.Wait() pattern, e.g. in NewBlockchain, NewTxPool.
• Remove unused function weightedCouncil.GetValidators()
  • The weightedCouncil.List() does the same job.

Types of changes

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• Bugfix
• New feature or enhancement
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