HBASE-24156: Switch the default registry to ZK based

In branch-2 we want to retain the ZK based registry as the default.

hbase-client/src/main/java/org/apache/hadoop/hbase/client/ConnectionRegistryFactory.java

@@ -36,7 +36,7 @@ final class ConnectionRegistryFactory {
    */
   static ConnectionRegistry getRegistry(Configuration conf) {
     Class<? extends ConnectionRegistry> clazz = conf.getClass(
-        CLIENT_CONNECTION_REGISTRY_IMPL_CONF_KEY, MasterRegistry.class,
+        CLIENT_CONNECTION_REGISTRY_IMPL_CONF_KEY, ZKConnectionRegistry.class,
         ConnectionRegistry.class);
     return ReflectionUtils.newInstance(clazz, conf);
   }