[Potential bug] Share instance in defaultOps

[kevindiu]

Describe the bug:

It have been found the same bug in 2 different package (tls and cache) so just in case I create a bug report for it.
Using defaultOps as var in option.go may create potential bugs in the program as it share the same instance to all callers.

I suggest to change all the implementation of defaultOps to from var to func()? (like this commit)
db434cb

To Reproduce:

Expected behavior:

Environment:

• Go Version: 1.14.3
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.11.5

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[kpango]

@kevindiu Could you please Investigate this Issue??