query tee proxy with support for comparison of responses

[sandeepsukhani]

What this PR does / why we need it:
Add querytee for proxying queries to 2 different loki instances. The code is reused from Cortex and more details about it can be found here. The only difference is it registers Loki specific routes and adds support for comparing log streams.

Special notes for your reviewer:
Routes are registered with GET method only because querytee didn't have support for registering routes with multiple method types. I have a PR in cortex to add support for it.
I will open a PR soon after updating Cortex vendor to add support for POST methods as well. Until then POST methods for querying won't work.

Checklist

• Tests updated

[cyriltovena]

LGTM - minus lint

[codecov-commenter]

Codecov Report

Merging #2211 into master will decrease coverage by 0.11%.
The diff coverage is 44.59%.

@@            Coverage Diff             @@
##           master    #2211      +/-   ##
==========================================
- Coverage   62.14%   62.02%   -0.12%     
==========================================
  Files         154      156       +2     
  Lines       12457    12531      +74     
==========================================
+ Hits         7741     7772      +31     
- Misses       4108     4149      +41     
- Partials      608      610       +2     

Impacted Files	Coverage Δ
cmd/querytee/main.go	0.00% <0.00%> (ø)
cmd/querytee/response_comparator.go	89.18% <89.18%> (ø)
pkg/querier/queryrange/downstreamer.go	95.87% <0.00%> (-2.07%)	⬇️