Change default compiler strategy

at our production ,when we have more than 20 agg expression, the compile time is high using the default janino. but when changed to jdk compiler,we gain fewer compile time than the janino one. Our product jdk version is 1.8.

So the default one should be JDK , if user's jdk version is upper than 1.7. We should add another check condition to the ClassCompilerSelector.