Problem: tx is not replaced by priority

[yihuang]

Closes: #1557

Solution:

• add TxReplacement callback, only replace if priority is higher

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Summary by CodeRabbit

• New Features

  • Improved transaction management by allowing higher-priority transactions to replace existing ones.

• Bug Fixes

  • Enhanced reliability of transaction processing through updated handling logic.

• Tests

  • Introduced a new test to validate transaction replacement behavior based on gas prices, ensuring integrity in transaction processing.