Broker skeleton

[shz117]

changes included:

• minor refactoring to prepare for broker implementation: memstore + aql context
• skeleton code for broker

TODOs are added including:

• implement BrokerSchemaMutator
• implement non agg query executor
• implement agg query executor
• implement different query options in handler
• logging and metrics where needed

[voyager-dw]

can we remove those duplicated stuff or create a new pull request?

[shz117]

can we remove those duplicated stuff or create a new pull request?

sure I will rebase on master since the cluster pkg is already checked in

[codecov-io]

Codecov Report

Merging #192 into master will decrease coverage by 0.03%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
- Coverage   74.55%   74.52%   -0.04%     
==========================================
  Files         137      137              
  Lines       18900    18909       +9     
==========================================
+ Hits        14091    14092       +1     
- Misses       3770     3777       +7     
- Partials     1039     1040       +1

Impacted Files	Coverage Δ
datanode/datanode.go	0% <ø> (ø)	:arrow_up:
memstore/table_shard.go	44.35% <ø> (ø)	:arrow_up:
query/aql_context.go	100% <ø> (ø)	:arrow_up:
memstore/host_memory_manager.go	96.81% <ø> (ø)	:arrow_up:
query/aql_postprocessor.go	80.31% <100%> (ø)	:arrow_up:
memstore/backfill_manager.go	87.12% <100%> (ø)	:arrow_up:
query/aql_compiler.go	76% <100%> (ø)	:arrow_up:
api/schema_handler.go	80.18% <100%> (ø)	:arrow_up:
api/debug_handler.go	74.42% <100%> (ø)	:arrow_up:
api/enum_handler.go	84.61% <100%> (ø)	:arrow_up:
... and 9 more

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