feat: Add Sample Locking + Massive Parallel test

[pboyd04]

I've added locking around the samples slice to avoid any race conditions. I could not cause the existing code to fail in anyway so I'm not really sure what the issue with the race condition was, but this should solve it.

[see-sharp-dev]

now the data race which I was talking about is visible in the failing tests

[pboyd04]

Man I really wish there was a way to label a race as "doesn't matter" because it really doesn't in this case, but fixed at any rate.

[see-sharp-dev]

thank you.
@hsluoyz can we merge this one? that will solve the issue with parallel test execution.

[hsluoyz]

@pboyd04 what issue does this PR fix? Better with an issue

[see-sharp-dev]

@hsluoyz we use Casbin enforcer, which under the hood uses this package.
after uptaking a recent version of this package parallel tests began to fail because of the detected race condition during unsynchronized access to the global variables.
similar was detected in previous test executions here

[github-actions]

🎉 This PR is included in version 1.7.0 🎉

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