Cleanup ORC chunked writer

[ttnghia]

This changes the internal variables of ORC chunked writer:

• Renaming them to have a _ prefix consistently.
• Add const qualifier to some variables that are writer parameters.
• Regroup them.

There is not any new implementation added. However, the unused parameter mr is removed from its interface thus this is flagged as breaking changes.

Closes:

• [FEA] Consistently use prefix/suffix for member variables of cudf::io::detail::orc::writer::impl #12973

[vyasr]

The improved consistency is good, but isn't the leading underscore a bit redundant in C++ where the private access specifier already conveys that these are internal variables?

[ttnghia]

The improved consistency is good, but isn't the leading underscore a bit redundant in C++ where the private access specifier already conveys that these are internal variables?

Consider the situations like this:

var1.member1 = func(_var2);

vs this:

var1.member1 = func(var2);

In the first situation, you can immediately recognize that we are changing a local variable by applying func on a member variable. On the other hand, the second situation can't tell you that. You have to dig into the code to find out what is var1 and var2 to understand it.

The underscore added to member variables is for the sheer clearness to help working on the code better, not to mean for any correctness.

[ttnghia]

/merge