Watch chan fails with "client connection closing" under stress test

[hongchaodeng]

After #6476 is fixed, rerunning the stress test for a long time (~1h) still gives out error:

rpc error: code = 9 desc = grpc: the client connection is closing

This is from the watch chan err response:

        for resp := range wch {
                if resp.Err() != nil {
                    t.Fatal(resp.Err())
                }
        ...

Script to keep test running:

while [ 1 ] ; do
  go test -v ./etcdbench -count 20 || break
done

[hongchaodeng]

cc @xiang90 @heyitsanthony

[heyitsanthony]

I added clus.Members[0].DropConnections() after the Watch in TestWatchOverlapContextCancel to stress the reconnect path with ctx cancelation and got unexpected closed channels. Probably related.