load startree index via segment reader interface

[klsince]

Loading startree index via SegmentReader interface to decouple from File objects, as done for the other index types.

[codecov-commenter]

Codecov Report

Merging #9828 (e4c0360) into master (4d16027) will increase coverage by 32.73%.
The diff coverage is 48.57%.

@@              Coverage Diff              @@
##             master    #9828       +/-   ##
=============================================
+ Coverage     28.14%   60.87%   +32.73%     
- Complexity       53     5391     +5338     
=============================================
  Files          1970     1970               
  Lines        106081   106101       +20     
  Branches      16089    16093        +4     
=============================================
+ Hits          29858    64594    +34736     
+ Misses        73294    36740    -36554     
- Partials       2929     4767     +1838     

Flag	Coverage Δ
integration1	?
integration2	24.53% <0.00%> (-0.04%)	⬇️
unittests1	67.95% <48.57%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ent/local/segment/store/FilePerIndexDirectory.java	85.04% <0.00%> (+85.04%)	⬆️
.../pinot/segment/spi/store/ColumnIndexDirectory.java	28.57% <0.00%> (+28.57%)	⬆️
...ache/pinot/segment/spi/store/SegmentDirectory.java	45.45% <0.00%> (+45.45%)	⬆️
.../local/segment/store/SingleFileIndexDirectory.java	89.80% <75.00%> (+89.80%)	⬆️
...indexsegment/immutable/ImmutableSegmentLoader.java	85.57% <100.00%> (+85.57%)	⬆️
...t/local/segment/store/SegmentLocalFSDirectory.java	73.41% <100.00%> (+73.41%)	⬆️
...ocal/startree/v2/store/StarTreeIndexContainer.java	100.00% <100.00%> (+100.00%)	⬆️
...local/startree/v2/store/StarTreeIndexMapUtils.java	85.00% <100.00%> (+85.00%)	⬆️
...pinot/minion/exception/TaskCancelledException.java	0.00% <0.00%> (-100.00%)	⬇️
...ot/broker/requesthandler/BrokerRequestHandler.java	0.00% <0.00%> (-100.00%)	⬇️
... and 1237 more

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[npawar]

lgtm