Fix race condition in CompareDataFrameStage

[dagardner-nv]

Description

• Fixes intermittent segfault in CompareDataFrameStage and ValidationStage

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[cwharris]

I wonder if get_messages should return a copy of the message array rather than an instance of it...

[dagardner-nv]

I wonder if get_messages should return a copy of the message array rather than an instance of it...

I considered that, but decided against it since:

1. It would come at a higher cost.
2. Calling clear should be self descriptive.

[dagardner-nv]

/merge