[FLINK-18709][Coordination] Implement PhysicalSlotProvider

[azagrebin]

What is the purpose of the change

PhysicalSlotProviderImpl tries to allocate a physical slot from the available idle cached slots in SlotPool.
If it is not possible, it requests a new slot from the SlotPool.

Brief change log

• Introduce interface PhysicalSlotProvider
• Implement it with the PhysicalSlotProviderImpl

Verifying this change

Unit tests.

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Last check on commit 881b186 (Wed Jul 29 09:44:08 UTC 2020)

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[zhuzhurk]

The change looks generally good to me.
Just some minor comments.

[azagrebin]

Thanks for the review @zhuzhurk
I have addressed the comments

[zhuzhurk]

Thanks for addressing all the comments @azagrebin