Make changes so that coherence works on a timing bus for the top-level of caches.

Make changes so that coherence works on a timing bus for the top-level of caches.
This added a snoopResponse callback to the caches, and a NACK to requests.

cpu/memtest/memtest.cc:
    Modified to work with do_events:
    No multiple requests to the same block outstanding at the same time from the same tester
    Using false sharing, each tester does only 1 byte access using it's id as the blk offset
    Allow for cycles before signaling deadlock, with do events it can take time to complete (NACK/blocked bus)
cpu/memtest/memtest.hh:
    Updated to keep an id with each tester (used for address generation)
    Updated to keep a list of outstanding address to prevent multiple outstanding per tester
    //Should really look into doing store forwarding within the tester, then we can test more functionality

--HG--
extra : convert_revision : 05fbcf547e4ffab9d220aeb73126ed787ca82239