code: public static ConnectionHolder getConnectionHolder(ConsumerBootstrap consumerBootstrap) {
          try {
              String connectionHolder = consumerBootstrap.getConsumerConfig().getConnectionHolder();
              ExtensionClass<ConnectionHolder> ext = ExtensionLoaderFactory
                  .getExtensionLoader(ConnectionHolder.class).getExtensionClass(connectionHolder);
              if (ext == null) {
                  throw ExceptionUtils.buildRuntime("consumer.connectionHolder", connectionHolder,
                      "Unsupported connectionHolder of client!");
              }
              return ext.getExtInstance(new Class[] { ConsumerBootstrap.class }, new Object[] { consumerBootstrap });
          } catch (SofaRpcRuntimeException e) {
              throw e;
          } catch (Throwable e) {
              throw new SofaRpcRuntimeException(e.getMessage(), e);
          }
      }
docs: 根据配置得到连接管理器
  
  @param consumerBootstrap 服务消费者配置
  @return ConnectionHolder