Retire memstore, user badger InMemory option instead

[bluesign]

Closes #83 and #202

Description

Very nice memory usage vs memstore.

For contributor use:

• Targeted PR against master branch
• Linked to GitHub issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the GitHub PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #203 (9a70cd4) into master (c3d473d) will increase coverage by 1.88%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
+ Coverage   49.52%   51.41%   +1.88%     
==========================================
  Files          30       29       -1     
  Lines        2441     2338     -103     
==========================================
- Hits         1209     1202       -7     
+ Misses       1088      991      -97     
- Partials      144      145       +1     

Flag	Coverage Δ
unittests	51.41% <46.66%> (+1.88%)	⬆️

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

Impacted Files	Coverage Δ
storage/badger/config.go	46.15% <33.33%> (-9.41%)	⬇️
blockchain.go	75.12% <50.00%> (-0.22%)	⬇️
server/server.go	50.78% <100.00%> (+0.78%)	⬆️
server/storage.go	50.00% <100.00%> (+44.28%)	⬆️
server/grpc.go	51.85% <0.00%> (-14.82%)	⬇️
server/admin.go	47.05% <0.00%> (-4.95%)	⬇️
server/rest.go	41.66% <0.00%> (+2.77%)	⬆️

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[sideninja]

Nice cleanup

[bluesign]

@turbolent @psiemens @janezpodhostnik Any chance to review this? I have couple of PRs accidentally branched on this one, if this is merged, will make my life much easier

[janezpodhostnik]

LGTM! Thank you!