Enable JSON compression

[shz117]

Enabling gzip on json response is beneficial.

• Great reduction in network usage (around 3x at 8MB), this should pay off even more when the cluster is under pressure, or payload is larger

• Throughput increase grows as payload size increases

  • ~30% boost at payload size ~8MB

• When payload is small, compressing actually increases size, but throughput not impacted much

• Under “BestSpeed” gzip option, CPU overhead seems neglectable.

• On per query level, avg response time reduced as well

[codecov[bot]]

Codecov Report

Merging #326 into master will decrease coverage by 0.05%. The diff coverage is 54.83%.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
- Coverage   72.03%   71.97%   -0.06%     
==========================================
  Files         167      167              
  Lines       23591    23624      +33     
==========================================
+ Hits        16994    17004      +10     
- Misses       5292     5317      +25     
+ Partials     1305     1303       -2

Impacted Files	Coverage Δ
utils/http.go	62.5% <ø> (ø)	:arrow_up:
api/common/response.go	0% <0%> (ø)	:arrow_up:
api/data_handler.go	70% <100%> (ø)	:arrow_up:
api/schema_handler.go	80.18% <100%> (ø)	:arrow_up:
datanode/client/query_client.go	66.66% <100%> (+0.43%)	:arrow_up:
api/enum_handler.go	84.61% <100%> (ø)	:arrow_up:
api/query_handler.go	73.68% <66.66%> (ø)	:arrow_up:
api/debug_handler.go	74.79% <89.47%> (ø)	:arrow_up:
memstore/batchstats.go	82.75% <0%> (-3.45%)	:arrow_down:
controller/mutators/etcd/schema_mutator.go	61.84% <0%> (-1.74%)	:arrow_down:
... and 9 more

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