Review and refactor synchronization handling between MockDirectoryWrapper and CheckIndex [LUCENE-10071]

[asfimport]

This is a spin-off issue from discussion in #128 as we noticed there's a subtle way to cause deadlock in test (or maybe even in production code if similar logic is implemented) #128 (comment) 

This issue is to review how synchronization can be improved between these classes to make it less deadlock-prone, or more explicit when locking arrangement needs to be made.

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Migrated from LUCENE-10071 by Zach Chen (@zacharymorn), updated Aug 28 2021

[asfimport]

Michael McCandless (@mikemccand) (migrated from JIRA)

Ideally we would be able to sometimes use concurrency inCheckIndex from MDW in tests.