mem: Add a requestor ID to each request object.

This change adds a requestor id to each request object which can be
used identify every device in the system that is capable of issuing a request. 
This is part of the way to removing the numCpus+1 stats in the cache and
replacing them with the requestor ids. This is one of a series of changes 
that make way for the stats output to be changed to python. 

no stats changes on regression tests