* Refactor TxPool: leaner and simpler
* Rewrite test_txpool
Reduce number of tables used, from 5 to 2. Reduce number of files.
If need to modify the price rule or other filters, now is far more easier because only one table to work with(sender/nonce).
And the other table is just a map from txHash to TxItemRef.
Removing transactions from txPool either because of producing new block or syncing became much easier.
Removing expired transactions also simple.
Explicit Tx Pending, Staged, or Packed status is removed. The status of the transactions can be inferred implicitly.
Developer new to TxPool can easily follow the logic.
But the most important is we can revive the test_txpool without dirty trick and remove usage of getCanonicalHead furthermore to prepare for better integration with ForkedChain.
7d3616e3d9