Add cache invalidation checks to BackupStoreDirectory.

Allows us to catch programming errors related to use of a cached reference
to a directory after the cache may have been invalidated by requesting another
directory from the cache, which could clear it.

In DEBUG builds, the cache is no longer cleared, but any entries in it are
invalidated, which is effectively a memory leak, but these builds should only
be used for running tests, where it doesn't matter.

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