Use gRPC instead of tchannel in hotrod

[pavolloffay]

Related to #2036
Related to #2129

This PR removes the dependency on tchannel-go from hotrod and instead uses gRPC. It also introduces https://github.com/opentracing-contrib/go-grpc dependency to trace the calls.

The number of reported spans is the same as before.

[codecov]

Codecov Report

Merging #2307 into master will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #2307      +/-   ##
==========================================
- Coverage   96.23%   96.21%   -0.02%     
==========================================
  Files         218      218              
  Lines       10692    10704      +12     
==========================================
+ Hits        10289    10299      +10     
- Misses        347      350       +3     
+ Partials       56       55       -1     

Impacted Files	Coverage Δ
cmd/query/app/server.go	93.18% <88.23%> (-1.63%)	⬇️
cmd/query/app/flags.go	100.00% <100.00%> (ø)
plugin/storage/badger/spanstore/reader.go	96.79% <0.00%> (ø)

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[objectiser]

@pavolloffay Is it possible to add a TLS testcase to improve the coverage?

[objectiser]

LGTM - would be good if the test coverage could be improved.

[pavolloffay]

The test coverage seems odd.

None of the following files are changed in this PR:

cmd/query/app/server.go | 93.18% <88.23%> (-1.63%) |  
-- | -- | --
cmd/query/app/flags.go | 100.00% <100.00%> (ø) |  
plugin/storage/badger/spanstore/reader.go

Anyways there are not tests in hotrod.