HPCC-32708 Various minor improvements to Thor disk read code

[ghalliday]

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Testing:

[ghalliday]

@jakesmith you may not like the changes (in the 3rd commit). I was walking through adding the generic disk reading code to thor, and struggling with the class structure a bit. This is some work to clean it up slightly.
For discussion to see if it is worthwhile.

[ghalliday]

(I would target the actual changes to master....)

[github-actions]

Jira Issue: https://hpccsystems.atlassian.net//browse/HPCC-32708

Jirabot Action Result:
Assigning user: [email protected]
Workflow Transition To: Merge Pending
Updated PR

[ghalliday]

@jakesmith this has been around for a while as a draft. I think it was some refactoring to start integrating parquet.
I'll re-review them myself, since I can't remember what the changes were. My memory is removing virtual calls and other minor optimizations.

[jakesmith]

@ghalliday - I haven't been able to spot any issues.