race can lead to multiple concurrent connection attempts to same target

[rade]

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diff --git a/router/connection.go b/router/connection.go
index e3967ec..810c419 100644
--- a/router/connection.go
+++ b/router/connection.go
@@ -360,6 +360,7 @@ func (conn *LocalConnection) shutdown(err error) {
                conn.forwarder.Stop()
        }

+       time.Sleep(1 * time.Minute)
        conn.Router.ConnectionMaker.ConnectionTerminated(conn.remoteTCPAddr, err
 }

and then

host1$ weave launch
host2$ weave launch host1
host1$ weave stop
host2$ weave status connections
-> 192.168.48.11:6783    failed      dial tcp4 192.168.48.11:6783: connection refused, retry: 2015-09-29 13:53:41.831462406 +0000 UTC
host2$ sleep 60
host2$ weave status connections
-> 192.168.48.11:6783    failed      read tcp4 192.168.48.11:6783: connection reset by peer, retry: 2015-09-29 13:55:09.21609408 +0000 UTC

This shows an incorrect change of the reported error - the "connection reset by peer" comes from the original failure when we stopped the other peer and stomps on the subsequent "connection refused" error.

However, with slightly different timing, namely the ConnectionTerminated occurring when there is an ongoing connection attempt, we can get multiple concurrent connection attempts to the same target. This breaks a pretty important invariant of the ConnectionMaker, so there may be other bad effects.