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dockershim: remove corrupt checkpoints immediately upon detection

Fixes https://github.com/kubernetes/kubernetes/issues/55620

The current checkpoint abstraction leaks the handling of corrupt checkpoints to the user.

If the user does a `GetCheckpoint()` and the checkpoint is corrupt, the corrupt checkpoint is returned to the user (why??) with an error indicating the corruption.  It is then up to the user to detect the corruption via the error msg and call `RemoveCheckpoint()` to remove the corrupted checkpoint.

The checkpoint abstraction should not expose corruption to the user.  If it is corrupt, it is as good as not found to the user.  The checkpoint code should handle cleanup of corrupt entries and report "not found" to the user.

@derekwaynecarr @eparis @dcbw @freehan