Allow star-tree creation during segment load

[Jackie-Jiang]

Description

Allow star-tree creation during segment load (and reload).
With this, star-tree can be added on the fly after updating the table config via reload.

NOTE:

• This feature only applies to adding star-tree index config and generate star-trees to the existing segments (modifying/removing star-tree index config is not supported now)
• Because the star-tree creation could consume a lot of system resources, added a boolean config enableDynamicStarTreeeCreation in IndexingConfig to enable/disable this feature (disabled by default).
• This feature should only be enabled for testing purpose or in cluster without too much load on the servers and with relatively small segments and simple star-tree index config (servers need to have free system resources to create the star-tree).

Other changes:

• Pass in enableDefaultStarTree into MultipleTreesBuilder instead of using null as a placeholder for default star-tree to enhance the readability
• Make MultipleTreesBuilder closeable to close the segment properly

Release Notes

Introduced a new boolean config enableDynamicStarTreeeCreation in IndexingConfig to enable/disable star-tree creation during segment load.

[mayankshriv]

Would be good to include the default value of this config (hopefully it is false). Also, as a user, if I think StarTree will help, what is the guideline for me to choose building it outside Pinot vs at load time (how do I know whether my production servers won't die if I enable this feature)?

[Jackie-Jiang]

@mayankshriv

• Boolean member variable is always default to false (e.g. we don't have an explicit default value for aggregateMetrics, createInvertedIndexDuringSegmentGeneration etc.)
• Star-tree is always created on the builder side. This feature is for users to add star-tree to the existing segments without star-tree. Added more descriptions in the PR.

[Jackie-Jiang]

@mayankshriv Can you please take another look?

[mayankshriv]

Please also update the docs to help users on choosing when to use this feature (specifically on how they can ensure that it won't put memory/cpu pressure on the server).