[sybild] add grpc service

[tmc]

This proto file describes the user/consumer oriented api. Node/leaf aggregation will likely need to include more detail than what is expressed here.

To do:

• [x] express sybil ingestion and query semantics
• [x] fully comment types and rpc endpoints
• [x] implement ListTables
• [x] implement GetTable
• [ ] implement Trim
• [x] implement Query
  • [x] samples
  • [x] counts
  • [x] sorting
  • [x] averages
  • [x] histograms
  • [x] filters
  • [x] time series
  • [x] replacements
  • [x] set field values
  • [x] ingestion log reading
• [x] implement Ingest
• [x] write snorkel driver?

[tmc]

I moved (start of) implementation bits to a separate branch to keep this focused on the proto file. (impl here: https://github.com/tmc/sybil/tree/sybild-implementation)

[tmc]

re: connection overhead - it's not strictly necessary to build a new connection per rpc call so that cost can be amortized.

[codecov[bot]]

Codecov Report

Merging #42 into grpc will decrease coverage by 8.78%. The diff coverage is 2.56%.

@@            Coverage Diff             @@
##             grpc      #42      +/-   ##
==========================================
- Coverage   44.67%   35.89%   -8.79%     
==========================================
  Files          48       53       +5     
  Lines        4407     5567    +1160     
==========================================
+ Hits         1969     1998      +29     
- Misses       2211     3341    +1130     
- Partials      227      228       +1

Flag	Coverage Δ
#fuzztests	0.98% <2.47%> (+0.39%)	:arrow_up:
#unittests	51.95% <6.06%> (-0.42%)	:arrow_down:

Impacted Files	Coverage Δ
src/cmd/cmd_aggregate.go	0% <0%> (ø)	:arrow_up:
src/sybil/printer.go	0% <0%> (ø)	:arrow_up:
src/cmd/cmd_ingest.go	0% <0%> (ø)	:arrow_up:
src/sybild/server.go	0% <0%> (ø)
src/sybild/ingest.go	0% <0%> (ø)
src/sybild/query.go	0% <0%> (ø)
src/cmd/cmd_query.go	0% <0%> (ø)	:arrow_up:
src/cmd/cmd_serve.go	0% <0%> (ø)
src/sybil/config.go	57.14% <100%> (+1.58%)	:arrow_up:
src/sybil/query_spec.go	57% <11.11%> (-4.96%)	:arrow_down:
... and 6 more

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[okayzed]

in master as https://github.com/logv/sybil/commit/364be19d8ddae6ecf25f07b629d93d42bb55a29b