Storage layer refactor & example new storage backend impl

[bluesign]

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Description

This is a proposed change to the storage layer, for which I drafted a simple version. We can discuss it, but it simplifies the storage layer and allows quickly adding new storage backends.

I implemented simple Redis server support for it (mostly related to #130 and similar requests ) redis/store.go is the simple implementation example for new backends.

Also, I want to move snapshot from badger DB to SQLite or something similar ( badger on windows is a bit problematic )

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For contributor use:

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

[bluesign]

I forgot to mention this will be a breaking change, as the storage format is changing a bit. We can make it backwards compatible and little ugly, or decide to break compatibility.

[devbugging]

Sorry for taking longer, but will soon try to review this 🙏

[devbugging]

@bluesign just make sure to resolve the conflict and we merge it.

[bluesign]

@sideninja can you assist me here a bit, I am lost in git :(

edit: I had to do force push, but I think required with rebase.

[codecov-commenter]

Codecov Report

Merging #221 (27cbc92) into master (dddefb8) will increase coverage by 1.45%. The diff coverage is 57.69%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   50.96%   52.42%   +1.45%     
==========================================
  Files          29       26       -3     
  Lines        2372     2041     -331     
==========================================
- Hits         1209     1070     -139     
+ Misses       1019      845     -174     
+ Partials      144      126      -18     

Flag	Coverage Δ
unittests	52.42% <57.69%> (+1.45%)	:arrow_up:

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

Impacted Files	Coverage Δ
server/server.go	48.50% <0.00%> (-0.74%)	:arrow_down:
server/storage.go	41.02% <0.00%> (-8.98%)	:arrow_down:
headers.go	21.42% <50.00%> (ø)
storage/store.go	54.54% <54.54%> (ø)
storage/encoding.go	82.60% <71.42%> (ø)
blockchain.go	74.83% <100.00%> (+0.04%)	:arrow_up:
blocks.go	60.00% <100.00%> (ø)
storage/badger/store.go
storage/badger/config.go

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