Transaction prioritization tests and hardening

[raulk]

Integration test:

1. Submit many transactions with high gas limit to stuff blocks and make the base fee spike to X or above.
2. Submit a transaction with gas fee cap way below X but above the minimum base fee.
3. Stop the flood so the base fee decreases, and ensure the transaction does not appear on chain until the base fee is below the gas fee cap.

Also:

1. Ensure we never accept transactions in the mempool below the minimum base fee (otherwise these txs can end up being queued forever).
2. Check what other clients like Lotus/Forest/Geth to deal with the upper end, if anything (e.g. too high premium, too high gas fee cap).

Originally posted by @raulk in #1185 (review)