Remove downsample flags from the query service

[joe-elliott]

Which problem is this PR solving?

• Fixes Downsampling in query service? #2727

Short description of the changes

• Splits the storage factory AddFlags method into two: AddFlags and AddFlagsPipeline with the latter meant to be used by Jaeger span pipeline processors (ingesters, distributors).
• Add appropriate tests

[codecov]

Codecov Report

Merging #2782 (1b6618d) into master (84b6f34) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2782      +/-   ##
==========================================
+ Coverage   95.83%   95.89%   +0.05%     
==========================================
  Files         217      217              
  Lines        9634     9640       +6     
==========================================
+ Hits         9233     9244      +11     
+ Misses        330      327       -3     
+ Partials       71       69       -2     

Impacted Files	Coverage Δ
plugin/storage/factory.go	98.18% <100.00%> (+0.10%)	⬆️
cmd/query/app/server.go	97.12% <0.00%> (+1.43%)	⬆️
cmd/query/app/static_handler.go	95.16% <0.00%> (+2.41%)	⬆️

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