BeanDefinitionParser implementation that parses "cluster" elements. It creates the cluster as well as a selector
that uses the criteria defined in the "selectors" sub-element to decide when the cluster must be selected for any
given EventListener.
The selector bean is defined using the name [cluster-id] + "$selector" and implements the
Ordered
interface
to define it's order relative to other selectors defined in the Spring Context.
If the cluster is defined as "default", another selector bean is defined under the name [cluster-id] +
"$defaultSelector", which also implements the
Ordered
interface, forcing it to be evaluated last of all.