feat(tx_pool): implement nonce gap handling

[MatusKysel]

Summary of Changes

1. Unified Eviction Logic

   • Merged two separate transaction eviction loops into a single optimized loop.
   • Handles eviction based on:
     • Account inactivity (pool.config.Lifetime).
     • Transaction age (txEvictionAge, default 24 hours, configurable).

2. Per-Account Nonce-Gap Limit

   • Introduced a limit (txMaxGapLimit = 32) on queued nonce-gap transactions per account.
   • Transactions exceeding this limit are automatically moved to the OverflowPool.

3. OverflowPool Integration

   • Updated eviction logic to transfer evicted transactions into the OverflowPool rather than discarding them immediately.
   • Enhanced logging to clearly indicate transactions moved to or from the OverflowPool.

4. Improved Transaction Reintroduction

   • Optimized logic for periodically transferring transactions from the OverflowPool back into the main pool, considering available slot capacity and transaction size.

5. Metrics and Monitoring Updates

   • Adjusted metrics to accurately track transactions evicted due to lifetime expiration or nonce-gap limits.

Motivation

• Efficiency: Streamlines transaction eviction and handling.
• Robustness: Provides better protection against potential DoS scenarios.
• Transaction Availability: Ensures better retention and reintroduction of transactions, reducing network overhead.
• Maintainability: Clarifies and simplifies transaction pool logic, enhancing readability and ease of maintenance.

Review Notes

• Validate combined eviction logic for correctness and concurrency handling.
• Confirm accuracy and effectiveness of the nonce-gap limiting.
• Ensure logging and metric changes are consistent and beneficial.