General cleanup principles:
* Use override when applicable.
* Use const on class members where possible.
* Renames where clarity can be improved.
* Qualify standard library names with std:: when possible, to
  distinguish from POSIX names.
* Qualify POSIX names with the global namespace (::) when possible, to
  distinguish from standard library names.

This also revamps the logic for putting together a message into the
in-memory buffer before that is passed to fwrite(). While correct in
practice, the current implementation advances a char pointer past the
size of its buffer, which is technically undefined behavior.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211472570