Dynamic service registration

This PR attempts to add two features.  Firstly, it allows one to register new types with DI even after the Autofac container is built.  This is done by calling the new `UpdateRegistrations()` method after registering.  Secondly, it adds a new overload to `RegisterService()` to allow for a new service operations to be added after app start.

The first feature is working, but the second one is not.  In any case, I want to merge this so that the contracts are in place.

@ZocDoc/coreinfrastructure