improve performance of getInstanceFromEnv

[liuzhao-lz]

Proposed changes

Improve performance of getInstanceFromEnv to avoid unnecessary synchronization

Branch to commit

• Branch kylin3 for v2.x to v3.x
• Branch kylin4 for v4.x
• Branch kylin5 for v5.x

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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Further comments

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[hit-lacus]

Why need add volatile here?

[liuzhao-lz]

非 volatile 变量的读取会从主存拷到寄存器，thread1 & thread2 进入 sychronized 前SYS_ENV_INSTANCE被拷贝到各自的寄存器，假如 thread2 先执行了 sychronized 内的代码，thread2执行完后 thread1 由于SYS_ENV_INSTANCE 拷贝到寄存器，第二次判空可能结果为 true，就会再一次执行 sychronized 内的代码

[hit-lacus]

LGTM