DS: Allow conditions for a class being available to the classloader

[kwin]

Sometimes DS implementations should only become active if certain classes can be loaded. Usually this is determined while the service is retrieved (i.e. BundleContext.getService() is being called). It would be beneficial to prevent the service registration altogether in that case (e.g. to allow a fallback implementation of the same service take its place).

[bjhargrave]

DS 1.5 will have Condition support. So anyone can author such a condition and make a component dependent upon it. I don't think we want to bake many different condition implementations into DS. DS will support the Condition primitive and then people can built upon it as they need.