HBASE-20895 NPE in RpcServer#readAndProcess

Synchronize readAndProcess() on the Connection to avoid MT issues with
data buffer management

hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RpcServer.java

@@ -1635,7 +1635,7 @@ public class RpcServer implements RpcServerInterface, ConfigurationObserver {
      * @throws IOException
      * @throws InterruptedException
      */
-    public int readAndProcess() throws IOException, InterruptedException {
+    public synchronized int readAndProcess() throws IOException, InterruptedException {
       // If we have not read the connection setup preamble, look to see if that is on the wire.
       if (!connectionPreambleRead) {
         int count = readPreamble();