stasis_state: Create internal stasis_state_proxy object.

This improves the way which stasis_state reference counting works.
Since manager->states holds onto the proxy object instead of the real
object this allows stasis_state objects to be freed when appropriate
without use of a special state_remove function.  Additionally each
distinct eid associated with the state holds a reference to the state to
prevent early release and potentially allow easier debug of leaks.

Change-Id: I400e0db4b9afa3d5cb4ac7dad60907897e73f9a9