The proxy API is the one that we would like to expose for applications
and the other API is used internally when implementing modules or
factories.
The current pw_core object is really a context for all objects so
name it that way. It also makes it possible to rename pw_core_proxy
to pw_proxy later.
Create an endpoint link object when linking endpoints. Keep track
of the links in the endpoint_link and cleanup when they are all
gone.
Improve properties on session objects.
