Fix [issue64].
Fix [issue64].

* Differentiate between DORPC_OK, DORPC_MARSHALLERR and
  DORPC_UNKNOWNNODE, with improved logging. Translate these into
  clnt_stats.
* Set alive to false if DORPC_UNKNOWNNODE.

Note, it's not clear to me that these fixes should go here instead of
vnode_impl::doRPC_cb.  Right now, vnode_impl::doRPC_cb just ignores
the error code translations.  Maybe location should be set_alive'd in
vnode_impl too...

weidezhang/dht,weidezhang/dht,sit/dht,weidezhang/dht,weidezhang/dht,sit/dht,sit/dht,weidezhang/dht,sit/dht,sit/dht