Fix JDK9 direct ByteBuffer cleaner invocation and initialize Cleaner0 when PlatformDependent0 is initialized.

Motivation:
The clean method in java.base/jdk.internal.ref.Cleaner is not accessible
to methods outside java.base.  This prevents Cleaner0.freeDirectBuffer
from actually calling the clean method on JDK9.

The issue could have been caught earlier if Cleaner0 is initialized when
PlatformDependent0 is initialized and logging statements in the static
initializer in Cleaner0 would be close to the logging statements in the
static initializer in PlatformDependent0.

Modifications:
Try casting the cleaner obtained from a ByteBuffer to Runnable and use
Runnable.run if possible. All Cleaners in JDK9 implements Runnable. Fall
back to the clean method if the cleaner does not implement Runnable.
The fallback preserves the behavior on JDK8 and earlier.

Try to free the direct ByteBuffer allocated during static initialization
of PlatformDependent0. This cause Cleaner0 to be initialized when
PlatformDependent0 is initialized, and logging statements from the
static initializers will be close together.

Result:

Cleaner0.freeDirectBuffer works as intended on JDK9 and logging shows
that Cleaner0.freeDirectBuffer works as intended.

common/src/main/java/io/netty/util/internal/Cleaner0.java

@@ -43,9 +43,15 @@ final class Cleaner0 {
             try {
                 cleanerField = direct.getClass().getDeclaredField("cleaner");
                 cleanerField.setAccessible(true);
-                Object cleaner = cleanerField.get(direct);
                 fieldOffset = PlatformDependent0.objectFieldOffset(cleanerField);
+                Object cleaner = cleanerField.get(direct);
+                try {
+                    // Cleaner implements Runnable from JDK9 onwards.
+                    Runnable runnable = (Runnable) cleaner;
+                    clean = Runnable.class.getDeclaredMethod("run");
+                } catch (ClassCastException ignored) {
                     clean = cleaner.getClass().getDeclaredMethod("clean");
+                }
                 clean.invoke(cleaner);
             } catch (Throwable t) {
                 // We don't have ByteBuffer.cleaner().

common/src/main/java/io/netty/util/internal/PlatformDependent0.java

@@ -154,6 +154,8 @@ final class PlatformDependent0 {
 
         logger.debug("java.nio.DirectByteBuffer.<init>(long, int): {}",
                 DIRECT_BUFFER_CONSTRUCTOR != null? "available" : "unavailable");
+
+        freeDirectBuffer(direct);
     }
 
     static boolean isUnaligned() {