Only add connection to ch.conns if it's active

If a connection becomes active, and then closes almost immediately, then
it's possible that the close is handled before onActive is done
processing which leads to adding a inactive connection to ch.conns.

This causes the test TestTimeoutCallsThenClose to fail with state
mismatch since the number of connections has changed.

Check the connection state before adding it to the channel to avoid this
race.

channel.go

@@ -541,6 +541,10 @@ func (ch *Channel) addConnection(c *Connection, direction connectionDirection) b
 	ch.mutable.Lock()
 	defer ch.mutable.Unlock()
 
+	if c.readState() != connectionActive {
+		return false
+	}
+
 	switch state := ch.mutable.state; state {
 	case ChannelStartClose:
 		// Outbound connections are allowed in ChannelStartClose, but the
@@ -572,6 +576,7 @@ func (ch *Channel) connectionActive(c *Connection, direction connectionDirection
 		c.log.WithFields(
 			LogField{"remoteHostPort", c.remotePeerInfo.HostPort},
 			LogField{"direction", direction},
+			ErrField(err),
 		).Warn("Failed to add connection to peer")
 	}