The free block list now gets populated on opening a database file.
Ideally we persist this list inside the heap itself, but for now this
prevents excessive heap growth.
Previously, only the first block in a chain of blocks would be
overwritten while all subsequent blocks would be appended to the heap.
Now we make sure to reuse all existing blocks in the chain.
Move the long storage test from TestSqlStatementExecution into a new
test unit called TestSqlHeap. Split it up into a flushed and non-flushed
variant so we test the write-ahead log as well.
