Functional Accesses: Update states to support Broadcast/Snooping protocols.

In the current implementation of Functional Accesses, it's very hard to implement broadcast or snooping protocols where the memory has no idea if it has exclusive access to a cache block or not. Without this knowledge, making sure the RW vs. RO permissions are right are next to impossible. So we add a new state called Backing_Store to enable the conveyance that this is the backup storage for a block, so that it can be written if it is the only possibly RW block in the system, or written even if there is another RW block in the ssytem.

Tested on a protocol on 10 random seeds to 1M accesses using ruby_mem_test.py with 25% functional accesses.