Enhance PubSub service to support multiple active contribution

It could be interesting to support multiple PubSub implementations at the same time.

A Nuxeo Stream impl based on Kafka could provide a fault tolerant mechanism.
An Hazelcast impl could provide an efficient PubSub for cache invalidation.

A simple way to do this without changing the interface is by adding a prefix to the contribution and a default impl.
This way topic with a prefix will point to an explicit contirbution.