ENH: return old file handler on change

- intended for the following type of use:

      auto oldHandler = fileHandler(fileOperation::NewUncollated());

      ... do something that only works with uncollated

      // Restore previous (if any)
      if (oldHandler)
      {
          fileHandler(std::move(oldHandler));
      }

ENH: make fileOperation distributed(bool) mutable

- use is "static-like" and akin to Pstream::parRun(bool),
  thus allow toggling of the switch without a const_cast