[TxPool] Fix duplicate transactions in account

[dbrajovic]

Description

This PR fixes the issue of two identical transactions ending up back-to-back inside an account. This bug is the true source of the numerous failed to apply tx: incorrect nonce errors.

The second transaction is a gossip of the first, but because indexing happens in handleEnqueueRequest both of them complete the preceding call to addTx bypassing the (now deprecated) check for ErrAlreadyKnown.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Additional comments

Fixes EDGE-585

[zivkovicmilos]

txpool: failed to apply tx: incorrect nonce, I banish you to the shadow realm 🚀

[Kourin1996]

LGTM