Added VRES type to the key of thread/VRES repository.

Added VRES type to the key of thread/VRES repository.

Since more than one single resource type has to be supported, it
came out that using only the thread-ID as the key of the trhead/VRES
repository (i.e., where the mapping between thread and vreses is kept)
is not enough.
In fact, what would happen if the user tries to bind _the_same_ thread
to two different vreses, each one associated to a different resource?

For that reason, the repository (and its accessor functions) has been
modified such that also the vres type is part of the key. Thus, the
same thread could be safely bound to one vres for each resource
we have in the system.