SinkMode Clean Up

This is a pull request for [issue #659](https://github.com/twitter/scalding/issues/659).

Changes
* Source now contains a sinkMode which defaults to SinkMode.REPLACE
* Let me know if this is wrong, but the SinkMode on a Tap should only affect the Tap used as a sink; ergo, there shouldn't be a need to conditionally check for the Access mode of read or write to force the the SinkMode to SinkMode.KEEP. So I ripped out the conditional checks.