[Core] Make Object Store memory default ratio env-configurable

[alexeykudinkin]

Currently, ratio of available memory allocated to Object Store is hard-coded at 30% which means that for most of the time Ray Data applications are actually not using 70% of memory.

This change

• Removes static cap of 200Gb on the Object Store size
• Makes DEFAULT_OBJECT_STORE_MEMORY_PROPORTION env-configurable

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[jjyao]

There is a comment for why having this cap: # Cap memory to avoid memory waste and perf issues on large nodes.

Can we leave this change out and just make the ratio configurable? Or also make this cap configurable.

[alexeykudinkin]

And that explanation isn't very useful, right? What's the waste? What are perf issues? No links, no context.

This cap of 200Gb is arbitrary and hard to explain for nodes that carry 1Tb+ memory available