Have moduleDescription method return const&
Have Worker use the module's ModuleDescription

The Service system used to guarantee that the address for a module's
ModuleDescription would be unique. When we allowed multiple Workers
per module that guarantee was broken. We now have the Workers use
the ModuleDescription from the module rather than one they hold.
Merge pull request #636 from Dr15Jones/useSameModuleDescription

Have Workers for the same module use the same ModuleDescription