[FLINK-1221] Use the source line as the default operator name

Before this change, we were using the class name for naming transformations.

With the change, we use the line of code where the dataset transformation is being called.
For example `org.apache.flink.api.java.operators.NamesTest.testJoinWith(NamesTest.java:92)` would be a name of a Join.

All this applies only if the user does not specify a name.