[#6712] Fix issue with jdk incompatibility in java RPC client

Summary:
See #6712 for
more details. The signature of ByteBuffer::flip changed between JDK 8 and 9. So when
the client is compiled using Java >8 and run with Java 8, it fails to
"link" the flip method and results in a NoSuchMethod error.

The fix converts the code to always use Buffer.flip instead of the
derived method. This should be not result in a loss of functionality
because the implementation of the flip in the derived class seems to be
the same as the parent class but just returns a different type. See
http://hg.openjdk.java.net/jdk9/sandbox/jdk/file/7d9bd66d2bb9/src/java.base/share/classes/java/nio/X-Buffer.java.template#l1148

The issue highlights another problem in the Java RPC client - it seems
like the error NoSuchMethodException is just swallowed somewhere along
the path. The 'sync' YBClient calls the AsyncYBClient but it might not
provide an 'errback' for the error that results from this call.

Test Plan:
Compile with Java 11, run with Java 8, verify the error is
fixed.

Reviewers: arnav, streddy, daniel

Reviewed By: streddy, daniel

Subscribers: streddy, yugaware

Differential Revision: https://phabricator.dev.yugabyte.com/D10183

java/yb-client/src/main/java/org/yb/util/Slices.java

@@ -41,6 +41,7 @@
 import com.google.common.base.Preconditions;
 import org.yb.annotations.InterfaceAudience;
 
+import java.nio.Buffer;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.nio.CharBuffer;
@@ -149,7 +150,8 @@ public static ByteBuffer encodeString(CharBuffer src, Charset charset)
     catch (CharacterCodingException x) {
       throw new IllegalStateException(x);
     }
-    dst.flip();
+    // See https://github.com/yugabyte/yugabyte-db/issues/6712
+    ((Buffer)dst).flip();
     return dst;
   }
 
@@ -171,7 +173,8 @@ public static String decodeString(ByteBuffer src, Charset charset)
     catch (CharacterCodingException x) {
       throw new IllegalStateException(x);
     }
-    return dst.flip().toString();
+    // See https://github.com/yugabyte/yugabyte-db/issues/6712
+    return ((Buffer)dst).flip().toString();
   }
 
   /**