mem: Create a request copy for deferred snoops

mem: Create a request copy for deferred snoops

Sometimes, we need to defer an express snoop in an MSHR, but the original
request might complete and deallocate the original pkt->req.  In those cases,
create a copy of the request so that someone who is inspecting the delayed
snoop can also inspect the request still.  All of this is rather hacky, but the
allocation / linking and general life-time management of Packet and Request is
rather tricky.  Deleting the copy is another tricky area, testing so far has
shown that the right copy is deleted at the right time.