[TxPool] Reject future transactions during high memory usage

[dbrajovic]

Description

This PR adds an additional constraint on transaction acquisition during periods of high slots usage.

If a transaction sent to the pool is not expected in terms of nonce (meaning it's a future transactions or spam) then it is rejected.

Changes include

• [ ] 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

Additional comments

Fixes EDGE-732

[codecov[bot]]

Codecov Report

Merging #690 (6e56e3e) into develop (210de9e) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #690      +/-   ##
===========================================
+ Coverage    48.72%   48.75%   +0.03%     
===========================================
  Files          112      112              
  Lines        15432    15437       +5     
===========================================
+ Hits          7519     7527       +8     
+ Misses        7279     7277       -2     
+ Partials       634      633       -1     

Impacted Files	Coverage Δ
txpool/txpool.go	72.68% <100.00%> (+0.28%)	:arrow_up:
txpool/account.go	94.94% <0.00%> (+1.68%)	:arrow_up:

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