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.

simple-db/src/java/simpledb/dbfile/HeapFile.java

Lines 154 to 157 in a029091

	public DbFileIterator iterator(TransactionId tid) {
	// some code goes here
	return new HeapFileIterator();
	}