public boolean isDurable(boolean defaultValue) {
		return getProperty(DURABLE, defaultValue);
	}
If true, subscriptions to taps/topics will be durable.
@param defaultValue the default value.
@return the property or default value.
spring-xd-messagebus-spi/src/main/java/org/springframework/xd/dirt/integration/bus/AbstractBusPropertiesAccessor.java