RecordUpdater should extend RecordWriter

A new interface ql.io.RecordUpdater was added as part of the ACID work in 0.13. This interface should extend RecordWriter because:

1. If it does not significant portions of FileSinkOperator will have to be reworked to handle both RecordWriter and RecordUpdater
2. Once a file format accepts transactions, it should not generally be possible to write using RecordWriter.write as that will write old style records without transaction information.