Implement Consul K/V storage backend

[dominikschulz]

Depends on #677

Fixes #672

[codecov]

Codecov Report

Merging #697 into master will decrease coverage by 0.03%.
The diff coverage is 68.09%.

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
- Coverage   64.41%   64.38%   -0.03%     
==========================================
  Files         150      150              
  Lines        8426     8450      +24     
==========================================
+ Hits         5427     5440      +13     
- Misses       2334     2344      +10     
- Partials      665      666       +1

Impacted Files	Coverage Δ
backend/store.go	100% <ø> (ø)	⬆️
backend/strings.go	80.65% <ø> (ø)	⬆️
backend/url.go	59.21% <0%> (-4.17%)	⬇️
store/sub/store.go	63.13% <76.19%> (+0.63%)	⬆️

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