Cleanup LOG.debug to make sure no extra toString and unnecessary String concatenation

The PR contains:
-) Remove unnecessary type check and cast for FileInputSplit in FileInputFormat.java.
-) Update LOG.debug to user SLF4J parameterized logging or protected by LOG.isDebugEnabled to save toString and String concat. I use SLF4J parameterized logging if it is just logging String or object via its toString and add LOG.isDebugEnabled if it requires calling method to get the message/object.

Pass tests.