Manually add transaction to Mempool

[brunoffranca]

Pull request checklist

• All tests pass. Demo project builds and runs.
• I have resolved any merge conflicts.

What's in this pull request?

1. Separated the transaction verification logic into its own function and simplified it.
2. Added a method to the Mempool to add a transaction manually.

[codecov]

Codecov Report

Merging #368 (0994256) into albatross (0c0b387) will increase coverage by 0.62%.
The diff coverage is 22.22%.

❗ Current head 0994256 differs from pull request most recent head bb2b2b0. Consider uploading reports for the commit bb2b2b0 to get more accurate results

@@              Coverage Diff              @@
##           albatross     #368      +/-   ##
=============================================
+ Coverage      29.81%   30.43%   +0.62%     
=============================================
  Files            346      347       +1     
  Lines          32930    32923       -7     
  Branches       15263    15267       +4     
=============================================
+ Hits            9818    10021     +203     
+ Misses         14268    14101     -167     
+ Partials        8844     8801      -43     

Flag	Coverage Δ
unittests	30.43% <22.22%> (+0.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mempool/src/executor.rs	0.00% <0.00%> (ø)
mempool/src/lib.rs	100.00% <ø> (ø)
mempool/src/mempool.rs	30.85% <0.00%> (+1.40%)	⬆️
primitives/transaction/src/lib.rs	45.75% <ø> (ø)
mempool/src/verify.rs	27.58% <27.58%> (ø)
database/src/volatile.rs	39.27% <0.00%> (-0.34%)	⬇️
primitives/block/src/block.rs	41.06% <0.00%> (ø)
utils/src/merkle/mod.rs	42.03% <0.00%> (+0.33%)	⬆️
beserial/src/lib.rs	48.85% <0.00%> (+0.38%)	⬆️
... and 39 more

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[jsdanielh]

Looks good to me. Only few minor comments