r3341: - don't zero the async structure (makes valgrind more useful)

r3341: - don't zero the async structure (makes valgrind more useful)

- get rid of req->mid, as it isn't a safe value to use to match
  requests in the server (it is safe in the client code, as we choose
  the mid, but in the server we can't rely on other clients to choose
  the mid carefully)
(This used to be commit 938fb44351e12a515073ea94cd306988d5ca7340)