HeapFile iterator must use {@link BufferPool#getPage}, rather than {@link #readPage} t

[jasonleaster]

Returns an iterator over all the tuples stored in this DbFile. The iterator must use {@link BufferPool#getPage}, rather than {@link #readPage} to iterate through the pages.

https://github.com/jasonleaster/simple-db/blob/a029091fa84f2074f50a202d66cccbc61dd0b625/src/java/simpledb/dbfile/HeapFile.java#L154-L157