Update of resource scheduler semantics

Update of resource scheduler semantics

The resource scheduler has been modified to have better support for mode
changes. It now offers two interfaces to implement the scheduler. For
simpler schedulers, there is a simple (one-vres) API, for more demanding
schedulers one callback (apply_vres_changes) for all changes is called.

VRES structure was also changed to allow the scheduler to better apply
the changes.