Review and refactor synchronization handling between MockDirectoryWrapper and CheckIndex

This is a spin-off issue from discussion in https://github.com/apache/lucene/pull/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) https://github.com/apache/lucene/pull/128#discussion_r642639399. 

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.