@EnableEurekaServer --->@Import(EurekaServerConfiguration.class)--->EurekaServerConfiguration的内部类EurekaServerConfigBeanConfiguration
注册serverConfig的实现类EurekaServerConfigBean，方法中有if (clientConfig.shouldRegisterWithEureka()) { server.setRegistrySyncRetries(5);}
所以registrySyncRetries的值根据shouldRegisterWithEureka的值有可能是5或者0