Add checkpoint index retention for multi entity detector

[weicongs-amazon]

Issue #, if available:

Description of changes:
Add checkpoint index retention logic for multi entity detectors.

Testing:

• UT - Done
• Functional testing in test domain - Done

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #283 (7c24c8b) into master (b1ec6b8) will decrease coverage by 1.00%.
The diff coverage is 91.86%.

@@             Coverage Diff              @@
##             master     #283      +/-   ##
============================================
- Coverage     73.22%   72.22%   -1.01%     
- Complexity     1749     1987     +238     
============================================
  Files           180      201      +21     
  Lines          8119     9547    +1428     
  Branches        670      847     +177     
============================================
+ Hits           5945     6895     +950     
- Misses         1864     2238     +374     
- Partials        310      414     +104     

Flag	Coverage Δ	Complexity Δ
plugin	71.64% <91.86%> (-0.99%)	0.00 <19.00> (ø)

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

Impacted Files	Coverage Δ	Complexity Δ
...endistroforelasticsearch/ad/cluster/DailyCron.java	100.00% <ø> (ø)	6.00 <0.00> (ø)
...relasticsearch/ad/cluster/MasterEventListener.java	94.59% <87.50%> (+0.15%)	8.00 <1.00> (ø)
...ter/diskcleanup/ModelCheckpointIndexRetention.java	89.47% <89.47%> (ø)	8.00 <8.00> (?)
...ticsearch/ad/cluster/diskcleanup/IndexCleanup.java	95.00% <95.00%> (ø)	10.00 <10.00> (?)
...rch/ad/transport/IndexAnomalyDetectorResponse.java	0.00% <0.00%> (-75.00%)	0.00% <0.00%> (-4.00%)
.../transport/SearchAnomalyResultTransportAction.java	48.38% <0.00%> (-29.40%)	4.00% <0.00%> (+2.00%)	⬇️
...ransport/SearchAnomalyDetectorTransportAction.java	48.38% <0.00%> (-29.40%)	4.00% <0.00%> (+2.00%)	⬇️
...sticsearch/ad/indices/AnomalyDetectionIndices.java	50.00% <0.00%> (-23.34%)	39.00% <0.00%> (+9.00%)	⬇️
...transport/IndexAnomalyDetectorTransportAction.java	76.00% <0.00%> (-16.00%)	6.00% <0.00%> (+3.00%)	⬇️
...search/ad/transport/GetAnomalyDetectorRequest.java	84.61% <0.00%> (-12.16%)	10.00% <0.00%> (+1.00%)	⬇️
... and 83 more

[ylwu-amzn]

LGTM. Thanks for the change!