Prevent loading ThreadLocalRandom in Java 6 environments (#138)

I'm not sure why but the JVM seems to load all classes which
are used in a (static) method. That makes it not safe to just
check the threadLocalRandomPresent flag and access
ThreadLocalRandom right away.

jaeger-core/src/main/java/com/uber/jaeger/utils/Java6CompatibleThreadLocalRandom.java

@@ -55,14 +55,24 @@ protected Random initialValue() {
    *
    * @return the current thread's {@link Random}
    */
-  @IgnoreJRERequirement
   public static Random current() {
     if (threadLocalRandomPresent) {
-      return ThreadLocalRandom.current();
+      return ThreadLocalRandomAccessor.getCurrentThreadLocalRandom();
     } else {
       return threadLocal.get();
     }
   }
 
+  /**
+   * This class prevents that {@link ThreadLocalRandom} gets loaded unless
+   * {@link #getCurrentThreadLocalRandom()} is called
+   */
+  private static class ThreadLocalRandomAccessor {
+    @IgnoreJRERequirement
+    private static Random getCurrentThreadLocalRandom() {
+      return ThreadLocalRandom.current();
+    }
+  }
+
   private Java6CompatibleThreadLocalRandom() {}
 }