New FileTransactionLogIterator implementation without read ahead

I renamed the previous implementation ReadAheadFileTransactionLogIterator. I am temporary keeping as reference and for testing but I intent to remove it ASAP in a future pull request. The new implementation is named NewFileTransactionLogIterator. I don't like that name and I want to simply name it FileTransactionLogIterator once ReadAheadFileTransactionLogIterator is gone. Meanwhile the FileTransactionLogIterator name is used by a common trait (extracted from ReadAhead/New implementations) and a companion object.