core/txpool: fix very high pending special transactions

[gzliudan]

Proposed changes

This PR fixes the bug of very high pending special transactions which is caused by the zero gas price.

When we call:

pending := w.eth.TxPool().Pending(true)

in the function commitNewWork(). The special transactions are filter out in the function Pending(enforceTips bool) because of zero gas price:

// Pending retrieves all currently processable transactions, grouped by origin
// account and sorted by nonce. The returned transaction set is a copy and can be
// freely modified by calling code.
//
// The enforceTips parameter can be used to do an extra filtering on the pending
// transactions and only return those whose **effective** tip is large enough in
// the next pending execution environment.
func (pool *TxPool) Pending(enforceTips bool) map[common.Address]types.Transactions {
	pool.mu.Lock()
	defer pool.mu.Unlock()

	pending := make(map[common.Address]types.Transactions)
	for addr, list := range pool.pending {
		txs := list.Flatten()

		// If the miner requests tip enforcement, cap the lists now
		if enforceTips && !pool.locals.contains(addr) {
			for i, tx := range txs {
				if !tx.IsSpecialTransaction() && tx.EffectiveGasTipIntCmp(pool.gasPrice, pool.priced.urgent.baseFee) < 0 {
					txs = txs[:i]
					break
				}
			}
		}
		if len(txs) > 0 {
			pending[addr] = txs
		}
	}
	return pending
}

Types of changes

What types of changes does your code introduce to XDC network?
Put an ✅ in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Regular KTLO or any of the maintaince work. e.g code style
• CICD Improvement

Impacted Components

Which part of the codebase this PR will touch base on,

Put an ✅ in the boxes that apply

• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

Put an ✅ in the boxes once you have confirmed below actions (or provide reasons on not doing so) that

• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A