BEP648: fix race condition in early finalization.

[zlacfzy]

Description

It's a fix for #3520
Fix a critical race condition in CheckFinalityAndNotify where the function relied on currentHeader matching targetBlockHash. Since currentHeader is updated later than highestVerifiedBlock in writeBlockAndSetHead, votes entering curVotes (via GetVerifiedBlockByHash check) would trigger CheckFinalityAndNotify before currentHeader was updated, causing the check to fail.

Rationale

In writeBlockAndSetHead, highestVerifiedBlock is set before currentHeader. When votes arrive and GetVerifiedBlockByHash returns non-nil, they enter curVotes and trigger CheckFinalityAndNotify. However, currentHeader hasn't been updated yet, so the check currentHeader.Hash() != targetBlockHash fails, preventing early finalization from working correctly.

Example

Before fix:

• CheckFinalityAndNotify skipped due to "currentHeader mismatch"
• Early finalization rarely triggered via VotePool

After fix:

• Target header retrieved directly by GetHeaderByHash
• Early finalization works reliably

Changes

Notable changes:

• CheckFinalityAndNotify: Use GetHeaderByHash(targetBlockHash) instead of
  checking currentHeader.Hash() == targetBlockHash
• VotePool: Add early filtering using HighestNotifiedFinal() to skip
  already-finalized blocks
• BlockChain: Add HighestNotifiedFinal() getter for deduplication
• Parlia: Remove finalizedNotified LRU cache