[TxPool] Introduce limit on enqueued transactions

[dbrajovic]

Description

This PR introduces a configurable limit on the amount of transactions any account in the TxPool can hold at any given time.

--max-enqueued [DEFAULT= 128]

As the enqueued part of an account just represents future transactions (higher nonce than expected), there are no particular benefits to keeping a large number of them in this queue as much as there are vulnerability risks that all result in an eventually bricked pool full of enqueued transactions.

The limit also enforces a rate limiting on transaction acquisition for any account - in effect, an incoming transaction would be rejected if there is no space for it in its respective enqueued queue. What this means is that aggressive spams are greatly mitigated, delaying the point of pool overflow.

Changes include

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

Checklist

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

Testing

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

Documentation update

Please link the documentation update PR in this section if it's present, otherwise delete it

Additional comments

Fixes EDGE-732

[codecov[bot]]

Codecov Report

Merging #687 (3f21675) into develop (de62840) will decrease coverage by 0.15%. The diff coverage is 23.33%.

@@             Coverage Diff             @@
##           develop     #687      +/-   ##
===========================================
- Coverage    48.77%   48.61%   -0.16%     
===========================================
  Files          112      112              
  Lines        15382    15390       +8     
===========================================
- Hits          7502     7482      -20     
- Misses        7249     7274      +25     
- Partials       631      634       +3     

Impacted Files	Coverage Δ
consensus/ibft/consensus_backend.go	0.00% <0.00%> (ø)
consensus/ibft/ibft.go	3.93% <0.00%> (-0.07%)	:arrow_down:
txpool/account.go	93.25% <100.00%> (+0.19%)	:arrow_up:
txpool/txpool.go	71.04% <100.00%> (ø)
network/server_discovery.go	73.88% <0.00%> (-15.93%)	:arrow_down:

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