Improve txpool_content performance for high-frequency polling

Summary
When polling txpool_content via the Ethereum JSON-RPC, I’m seeing a hard ceiling of ~200 transactions per second retrieved, even when the node’s actual mempool is much larger and incoming rate is higher.

This is likely due to the heavy JSON serialization and full-pool traversal in each RPC call. The endpoint works fine for occasional inspection, but is too slow for real-time monitoring or ingestion scenarios that require higher throughput.

Reproduction
1. Run reth with default RPC settings and a live mainnet sync.
2. Call txpool_content repeatedly (HTTP or WS eth_call) as fast as possible from a single client.
3. Measure transactions retrieved per second — result is consistently ~200 tx/s even with thousands of txs pending in the pool.

Expected behavior
• Ability to retrieve higher throughput from the mempool via RPC (e.g., closer to the actual incoming tx rate), especially for local/IPC clients.
• Option to fetch incremental changes instead of the full pool every time.

Why this matters
• Full-pool polling is currently the only way to get a complete snapshot of pending and queued txs via JSON-RPC.
• Many applications (analytics, MEV research, relays, custom ordering engines) need near-real-time mempool state but can’t efficiently use txpool_content due to this throughput cap.

Potential solutions
1. Add a streaming or incremental API — e.g., a txpool_diff or subscription that streams full tx bodies for new/removed txs (similar to eth_subscribe:newPendingTransactions but with bodies, not just hashes).
2. Expose a binary/IPC interface — avoid JSON encoding overhead for local clients.
3. Allow partial fetches — parameters to limit to N txs, specific senders, or time ranges to reduce traversal cost.
4. Optimize serialization — e.g., pre-encode pool entries to JSON or use more efficient traversal for RPC dumps.

Bắt đầu bằng cách tái hiện các lệnh gọi txpool_content lặp lại qua HTTP và WS eth_call trên một quá trình đồng bộ mainnet đang chạy, đo số giao dịch lấy được mỗi giây và kích thước pool. So sánh chi phí duyệt toàn bộ pool và tuần tự hóa JSON được mô tả trong issue. Hoàn thành có nghĩa là đã triển khai một phương pháp cụ thể có throughput cao hơn hoặc truy xuất gia tăng, đồng thời đo hiệu năng của phương pháp đó so với giới hạn ~200 tx/s được báo cáo.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.