[NLJ] implementation layout

[shz117]

• add comments for existing many to one join logic
• add TODOs for code change to support NLJ
• add query context fields needed for NLJ
• compiler changes for matching many to many joins

[codecov[bot]]

Codecov Report

Merging #299 into master will increase coverage by 0.04%. The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
+ Coverage   71.72%   71.76%   +0.04%     
==========================================
  Files         166      166              
  Lines       23137    23152      +15     
==========================================
+ Hits        16594    16615      +21     
+ Misses       5249     5243       -6     
  Partials     1294     1294

Impacted Files	Coverage Δ
query/aql_context.go	88.23% <ø> (ø)	:arrow_up:
query/aql_processor.go	79.88% <ø> (ø)	:arrow_up:
query/time_series_aggregate.go	77.37% <ø> (ø)	:arrow_up:
query/aql_batchexecutor.go	82.53% <100%> (ø)	:arrow_up:
query/aql_compiler.go	74.27% <90.47%> (+0.53%)	:arrow_up:
controller/mutators/etcd/schema_mutator.go	63.58% <0%> (+1.73%)	:arrow_up:

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[voyager-dw]

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