Fix mvn exec:java ClassNotFoundException (cmu-db#493)

Running benchmarks without packaging first fails like so:

```
mvn clean compile exec:java -P postgres -Dexec.args="-b tpcc -c config/postgres/sample_tpcc_config.xml --create=true --load=true --execute=true"

java.lang.RuntimeException: Failed to retrieve class for com.oltpbenchmark.benchmarks.tpcc.TPCCBenchmark
    at com.oltpbenchmark.util.ClassUtil.getClass (ClassUtil.java:200)
    at com.oltpbenchmark.util.ClassUtil.getClass (ClassUtil.java:187)
    at com.oltpbenchmark.util.ClassUtil.newInstance (ClassUtil.java:112)
    at com.oltpbenchmark.DBWorkload.main (DBWorkload.java:165)
    at org.codehaus.mojo.exec.ExecJavaMojo.doMain (ExecJavaMojo.java:385)
    at org.codehaus.mojo.exec.ExecJavaMojo.doExec (ExecJavaMojo.java:374)
    at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0 (ExecJavaMojo.java:296)
    at java.lang.Thread.run (Thread.java:1583)
Caused by: java.lang.ClassNotFoundException: com.oltpbenchmark.benchmarks.tpcc.TPCCBenchmark
    at jdk.internal.loader.BuiltinClassLoader.loadClass (BuiltinClassLoader.java:641)
    at jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass (ClassLoaders.java:188)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:526)
    at java.lang.Class.forName0 (Native Method)
    at java.lang.Class.forName (Class.java:534)
    at java.lang.Class.forName (Class.java:513)
    at org.apache.commons.lang3.ClassUtils.getClass (ClassUtils.java:532)
    at org.apache.commons.lang3.ClassUtils.getClass (ClassUtils.java:514)
    at com.oltpbenchmark.util.ClassUtil.getClass (ClassUtil.java:198)
    at com.oltpbenchmark.util.ClassUtil.getClass (ClassUtil.java:187)
    at com.oltpbenchmark.util.ClassUtil.newInstance (ClassUtil.java:112)
    at com.oltpbenchmark.DBWorkload.main (DBWorkload.java:165)
    at org.codehaus.mojo.exec.ExecJavaMojo.doMain (ExecJavaMojo.java:385)
    at org.codehaus.mojo.exec.ExecJavaMojo.doExec (ExecJavaMojo.java:374)
    at org.codehaus.mojo.exec.ExecJavaMojo.lambda$execute$0 (ExecJavaMojo.java:296)
    at java.lang.Thread.run (Thread.java:1583)
```

The bug was introduced by 611f3d4. I believe we should be using
`Thread.getCurrentThread().getContextClassLoader()`, not
`ClassLoader.getSystemClassLoader()`.

src/main/java/com/oltpbenchmark/api/BenchmarkModule.java

@@ -62,15 +62,15 @@ public BenchmarkModule(WorkloadConfiguration workConf) {
     this.workConf = workConf;
     this.dialects = new StatementDialects(workConf);
     // setClassLoader();
-    this.classLoader = ClassLoader.getSystemClassLoader();
+    this.classLoader = Thread.currentThread().getContextClassLoader();
   }
 
   /**
    * Instantiates the classLoader variable, needs to be overwritten if benchmark uses a custom
    * implementation.
    */
   protected void setClassLoader() {
-    this.classLoader = ClassLoader.getSystemClassLoader();
+    this.classLoader = Thread.currentThread().getContextClassLoader();
   }
 
   // --------------------------------------------------------------------------

src/main/java/com/oltpbenchmark/util/ClassUtil.java

@@ -184,7 +184,7 @@ public static <T> Constructor<T> getConstructor(Class<T> target_class, Class<?>.
    * @return
    */
   public static Class<?> getClass(String class_name) {
-    return getClass(ClassLoader.getSystemClassLoader(), class_name);
+    return getClass(Thread.currentThread().getContextClassLoader(), class_name);
   }
 
   /**