feat(cli): add experimental db export/import commands

[jansegre]

This PR depends on #410, it's basically two commands (prefixed with x- to indicate they are experimental): x-import and x-export, that will export raw blocks+transactions with no metadata with a trivial serialization. The format is intended to be simple enough so it can be stable, it's useful for recreating a database without having to sync to the network, and also useful for running experiments related to the loading of transactions.

[codecov]

Codecov Report

Merging #413 (0938453) into dev (d0db4c1) will decrease coverage by 0.24%.
The diff coverage is 10.34%.

❗ Current head 0938453 differs from pull request most recent head decae0a. Consider uploading reports for the commit decae0a to get more accurate results

@@            Coverage Diff             @@
##              dev     #413      +/-   ##
==========================================
- Coverage   82.87%   82.62%   -0.25%     
==========================================
  Files         171      171              
  Lines       15613    15671      +58     
  Branches     2470     2476       +6     
==========================================
+ Hits        12939    12948       +9     
- Misses       2227     2277      +50     
+ Partials      447      446       -1     

Impacted Files	Coverage Δ
hathor/util.py	63.67% <10.34%> (-17.58%)	⬇️
hathor/indexes/manager.py	92.85% <0.00%> (-0.90%)	⬇️
hathor/p2p/node_sync.py	84.02% <0.00%> (+0.98%)	⬆️

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[msbrogli]

I started reviewing it but I noticed that it has changes from other PRs. I'll wait for a rebase to finish the review.

[jansegre]

Last test runs were OK, this run just has the commits signed.