Return MINIRPC_NETWORK_FAILURE from sender-sync calls if squashing events
Return MINIRPC_NETWORK_FAILURE from sender-sync calls if squashing events

- After mrpc_conn_close(), return MINIRPC_NETWORK_FAILURE from
sender-synchronous RPCs whose replies arrive on the fd while it is still
open.  Note that there is a window after the mrpc_conn_close() returns in
which we still may return the actual reply, due to an unavoidable race.

cmusatyalab/minirpc,cmusatyalab/minirpc