Create a parallel version to support JDK14/15 (mostly for off-heap memory management)

[sanastas]

Base all of the off-heap memory management on new Java14/15 VarHandle API, without unsafe usage. Other than memory management, the rest of the code should be easily transferred to JDK14. The reason for the change is to allow much wider using for Oak.

Implementing this issue requires a significant code writing: new (alternative) Block, BlockPool, MemoryAllocator, MemoryManager etc.

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