Provide option to clean up DFS sources after each commit

Since DeltaStreamer makes heavily use of file listing, if the source contains a lot of tiny files, this could  quickly become a bottle neck. We need a way to delete/archive files once processed by DeltaStreamer. 

It seems like the best way to reliably clean up the source is after DeltaSync commit the checkpoint successfully. We could add a new public method to Source e.g. `postCommit()` and invoke it after each successful commit 

Reference:

https://spark.apache.org/docs/latest/structured-streaming-programming-guide.html#input-sources