[FLINK-3382] Improve clarity of object reuse in ReusingMutableToRegularIteratorWrapper

[greghogan]

No description provided.

[ggevay]

Shouldn't the current record remain valid if hasNext() returned true? I mean the user might be holding on to the object returned in next, and expect it to not be changed by a hasNext call:

T cur = it.next();
if(it.hasNext()) {
  // here, I would expect cur to not have changed since the next() call
}

[greghogan]

Ah, yes, now I see. I'll just burn this.