Disk buffering iterator explicit removal

[LikeTheSalad]

Addresses #2540 by not implicitly removing the items from disk during the reading iteration.

This is a breaking change as now users are expected to remove the disk items explicitly by calling Iterator.remove(). For example:

Iterator<Collection<SpanData>> spanIterator = spanStorage.iterator();
while (spanIterator.hasNext()) {
  Collection<SpanData> spans = spanIterator.next();
  // ...
  spanIterator.remove(); // This line must be added now to keep the existing functionality.
}