respect data only mode

[shz117]

DataOnly flag is set to true means the following:

• the query is a "distributed mode" query, or,
• it's sent from aresdb-broker

in this mode, we do several special things compared to single instance mode:

• for non agg query, we ignore response header(the column name list) and stream out only the individual rows, as the headers and packaging to json object will be handled in the broker
• for both non agg and agg queries, we skip enum translation, as in distributed mode the source of truth of enums is in the controller, and broker will cache it in memory and be responsible for translating enum in the post processing phase

[codecov-io]

Codecov Report

Merging #278 into master will increase coverage by 0.02%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage    71.8%   71.83%   +0.02%     
==========================================
  Files         166      166              
  Lines       22868    22870       +2     
==========================================
+ Hits        16421    16429       +8     
+ Misses       5167     5163       -4     
+ Partials     1280     1278       -2

Impacted Files	Coverage Δ
query/aql_postprocessor.go	81.48% <100%> (+0.27%)	:arrow_up:
memstore/cuckoo_index.go	91.27% <0%> (+1%)	:arrow_up:
broker/query_plan_non_agg.go	86.56% <0%> (+2.23%)	:arrow_up:

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