commandserver: handle backlog before exiting Previously, when a chg server is exiting, it does not handle connected clients so clients may get ECONNRESET and crash: 1. client connect() # success 2. server shouldexit = True and exit 3. client recv() # ECONNRESET d7875bfbfccb makes this race condition easier to reproduce if a lot of short chg commands are started in parallel. This patch fixes the above issue by unlinking the socket path to stop queuing new connections and processing all pending connections before exit.