Another pass of minor changes in preparation for new protocol.

Another pass of minor changes in preparation for new protocol.

src/mem/cache/cache_impl.hh:
src/mem/cache/coherence/simple_coherence.hh:
    Get rid of old invalidate propagation logic in preparation
    for new multilevel snoop protocol.
src/mem/cache/coherence/coherence_protocol.cc:
    L2 cache now has protocol, so protocol must handle ReadExReq
    coming in from the CPU side.
src/mem/cache/miss/mshr_queue.cc:
    Assertion is failing, so let's take it out for now.
src/mem/packet.cc:
src/mem/packet.hh:
    Add WritebackAck command.
    Reorganize enum to put responses next to corresponding requests.
    Get rid of unused WriteReqNoAck.

--HG--
extra : convert_revision : 24c519846d161978123f9aa029ae358a41546c73