Synchronize class generation to prevent corruption

```java
java.lang.ClassFormatError: Extra bytes at the end of class file Q_01686498692_02056817302_032091036
 at java.lang.ClassLoader.defineClass1(Native Method)
 at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
 at java.lang.ClassLoader.defineClass(ClassLoader.java:643)
 at com.mysema.codegen.MemClassLoader.findClass(MemClassLoader.java:60)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
 at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
 at com.mysema.codegen.AbstractEvaluatorFactory.createEvaluator(AbstractEvaluatorFactory.java:130)

 at com.mysema.codegen.AbstractEvaluatorFactory.createEvaluator(AbstractEvaluatorFactory.java:94)
 at com.mysema.query.collections.DefaultEvaluatorFactory.create(DefaultEvaluatorFactory.java:125)
 at com.mysema.query.collections.DefaultQueryEngine.project(DefaultQueryEngine.java:234)
 at com.mysema.query.collections.DefaultQueryEngine.evaluateSingleSource(DefaultQueryEngine.java:192)
 at com.mysema.query.collections.DefaultQueryEngine.list(DefaultQueryEngine.java:91)
 at com.mysema.query.collections.AbstractCollQuery.list(AbstractCollQuery.java:202)
```
can occur when multiple threads attempt to generate classes simultaneously.

Link to original issue: https://groups.google.com/forum/#!topic/querydsl/ud4ow-siCn8