Store: Update preTransactionFileSize after each call to commit()

[clarisma]

If there is more than one commit in a transaction, blocks appended beyond preTransactionFileSize during a previous commit phase are not journaled; if a later phase in the same transaction modifies these blocks, but fails before completing commit(), partial modifications may not be rolled back.

Solution: Rename preTransactionFileSize to preCommitFileSize, and update after each call to commit().