Refactor IDataProducer hierarchy

[alexrobin]

Refactoring is needed to allow:

• A module to create multiple data producers => separation of IModule interface from IDataProducer interface
• Handle multi-producers more cleanly => redesign IMultiProducer as a simple group of IDataProducer

As a consequence, SOS and stream storage need to be updated

[alexrobin]

Done in b7ad670