Get rid of reflect in mempool code

[tzdybal]

This PR copies the mempool dependency into code tree and integrate the changes, to avoid using reflect in mempool related code.

Depends on #34.
Relates to #37.
Resolves#38.

[liamsi]

What happens if the cast fails?

[tzdybal]

We will see a panic in tests and will be able to fix the issue.
Initially I created idiomatic error handling for this cast... But in this particular case I think we should just panic.

[liamsi]

I see, I briefly thought this was the other way around: you could send a Tx and crash the process / node but in this case the process is in control.

[liamsi]

It would be helpful for reviewers to highlight which files were and how they were modified in github comments.

[codecov-io]

Codecov Report

Merging #39 (a33ee44) into main (2ecdb45) will increase coverage by 7.86%.
The diff coverage is 69.64%.

@@ Coverage Diff @@## main #39 +/- ##
==========================================
+ Coverage 51.68% 59.54% +7.86% 
==========================================
Files 6 10 +4 Lines 416 749 +333 ==========================================
+ Hits 215 446 +231 - Misses 168 251 +83 - Partials 33 52 +19 

Impacted Files	Coverage Δ
rpcclient/local.go	27.16% <ø> (ø)
mempool/metrics.go	15.15% <15.15%> (ø)
mempool/errors.go	25.00% <25.00%> (ø)
mempool/clist_mempool.go	76.53% <76.53%> (ø)
mempool/mempool.go	81.25% <81.25%> (ø)
node/node.go	66.19% <100.00%> (-0.47%)	⬇️

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