Move the iterator to the next memory block. The next memory block starts at the first element
that was not in the block before. A special case is when no record was in the block before,
which happens when this function is invoked two times directly in a sequence, without calling
hasNext() or next in between. Then the block moves one element.
True if a new memory block was loaded, false if there were no further records and
hence no further memory block.
IOException - Thrown, when advancing to the next block failed.