Fix the (potential) race-condition issue on `IgnorableColumns`

What does this MR do?

Since the Ruby threads are preemptive on the user level, the scheduler can decide to do the context switching to give the execution to another thread after the conditional check. If this happens, two threads will generate the exact same object. We can avoid this by just synchronizing the whole block.

Since the Mutex is not reentrant in Ruby, I've also changed the way we synchronize the critical section with Monitor.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
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Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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