Too many class loaders created during jitting by the ClassGenerator

The current implementation of the ClassGenerator causes the following classloading leaks:

• org.drools.rule.builder.dialect.asm.ClassGenerator.EMPTY_METHOD_BODY is a static instance which will hold to the last instance of InternalClassLoader created

• org.drools.rule.constraint.MvelConstraint uses an ExecutorHolder that spawns daemon threads and keeps them in a pool. Problem is these threads create class loaders, and can only be garbage collected when the thread dies