hide the synchronization inside these methods

so we don't get warnings about the overriding methods also needing to be
synchronized, which is not the case

h2/src/main/org/h2/store/fs/FileBase.java

@@ -34,24 +34,28 @@ public abstract class FileBase extends FileChannel {
     public abstract int write(ByteBuffer src) throws IOException;
 
     @Override
-    public synchronized int read(ByteBuffer dst, long position)
+    public int read(ByteBuffer dst, long position)
             throws IOException {
+        synchronized(this) {
             long oldPos = position();
             position(position);
             int len = read(dst);
             position(oldPos);
             return len;
         }
+    }
 
     @Override
-    public synchronized int write(ByteBuffer src, long position)
+    public int write(ByteBuffer src, long position)
             throws IOException {
+        synchronized(this) {
             long oldPos = position();
             position(position);
             int len = write(src);
             position(oldPos);
             return len;
         }
+    }
 
     @Override
     public abstract FileChannel truncate(long size) throws IOException;