Enable CI for badger and grpc-plugin

[yurishkuro]

Partially address #1516 since the badger and grpc-plugin storage integration tests are not running in CI at all today.

• adds a new CI step to run grpc-plugin and badger integration tests
• minor clean-up of the plugin framework code
• reorder logic in .travis.yml to match the actual order of the matrix steps

[yurishkuro]

cc @albertteoh

[codecov]

Codecov Report

Merging #2474 into master will increase coverage by 0.03%.
The diff coverage is 7.69%.

@@            Coverage Diff             @@
##           master    #2474      +/-   ##
==========================================
+ Coverage   95.50%   95.54%   +0.03%     
==========================================
  Files         208      208              
  Lines       10750    10750              
==========================================
+ Hits        10267    10271       +4     
+ Misses        407      404       -3     
+ Partials       76       75       -1     

Impacted Files	Coverage Δ
plugin/storage/grpc/config/config.go	0.00% <ø> (ø)
plugin/storage/grpc/shared/plugin.go	0.00% <0.00%> (ø)
plugin/storage/grpc/factory.go	100.00% <100.00%> (ø)
cmd/flags/admin.go	77.77% <0.00%> (-1.59%)	⬇️
plugin/storage/integration/integration.go	80.86% <0.00%> (+0.47%)	⬆️
...lugin/sampling/strategystore/adaptive/processor.go	100.00% <0.00%> (+0.79%)	⬆️
cmd/query/app/server.go	90.69% <0.00%> (+1.55%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a8e83cd...b20419d. Read the comment docs.