Fix JDK 11 code-level incompatibilities

sun.nio.ch.DirectBuffer.cleaner() is of type jdk.internal.ref.Cleaner on
JDK9+. It's also not accessible due to tighter encapsulation without
special handling like --add-opens=java.base/jdk.internal.ref=ALL-UNNAMED

Author: GreyTeardrop

lab-core/src/main/java/com/github/jnthnclt/os/lab/core/io/DirectBufferCleaner.java

@@ -24,12 +24,13 @@ class DirectBufferCleaner {
         try {
             _directBufferClass = Class.forName("sun.nio.ch.DirectBuffer");
             _directBufferCleanerMethod = _directBufferClass.getMethod("cleaner");
-            _cleanerClass = Class.forName("sun.misc.Cleaner");
+            _cleanerClass = _directBufferCleanerMethod.getReturnType();
             _cleanMethod = _cleanerClass.getMethod("clean");
             _available = true;
         } catch (ClassNotFoundException | NoSuchMethodException | SecurityException e) {
-            System.out.println("Failed to reflect direct buffer cleaner, these methods will be unavailable");
-            e.printStackTrace();
+            System.out.println("Failed to reflect direct buffer cleaner, these methods will be unavailable. " +
+                               "If you are on Java 9+ add --add-opens=java.base/jdk.internal.ref=ALL-UNNAMED " +
+                               "to JVM options");
         }
         directBufferClass = _directBufferClass;
         directBufferCleanerMethod = _directBufferCleanerMethod;
@@ -46,8 +47,9 @@ static public void clean(ByteBuffer bb) {
                     cleanMethod.invoke(cleaner);
                 }
             } catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
-                System.out.println("Failed to clean buffer");
-                e.printStackTrace();
+                System.out.println("Failed to clean buffer. " +
+                                   "If you are on Java 9+ add --add-opens=java.base/jdk.internal.ref=ALL-UNNAMED " +
+                                   "to JVM options");
             }
         }
     }

lab-core/src/test/java/com/github/jnthnclt/os/lab/core/LABValidationNGTest.java

@@ -325,7 +325,7 @@ private void validationTest(LAB lab) throws InterruptedException, Exception, Exe
                                 },
                                 (index, key, timestamp, tombstoned, version1, value1) -> {
                                     hits.incrementAndGet();
-                                    found.add(key == null ? 0 : key.getLong(0));
+                                    found.add(key == null ? 0L : key.getLong(0));
                                     return true;
                                 }, true);
                         }