Tpetra: Address "FIXME" comments in FixedHashTable

[tasmith4]

Enhancement

@trilinos/tpetra

While getting FixedHashTable running without UVM, in some cases we took the shortest route to get running quickly, and need to go back to think more deeply about some design choices.

FIXME comments have been left to highlight areas to reinvestigate.

See also #8267 and #8104

[github-actions]

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