openClientByURL: Fix for >1 master (split URL host)

In a NEO URI more than one master node can be specified, because a NEO
cluster may have more masters than one. But before this patch
'openClientByURL' always assumed that the given URL only specifies one
master. Now the host is split into potentially > 1 master nodes. It
therefore works now in the same way as the Python implementation [1].

[1] https://lab.nexedi.com/nexedi/neoppod/blob/342168cd/neo/client/zodburi.py#L64

go/neo/client.go

@@ -483,7 +483,7 @@ func openClientByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (
 		return nil, zodb.InvalidTid, err
 	}
 
-	c := NewClient(name, []string{u.Host}, net)
+	c := NewClient(name, strings.Split(u.Host, ","), net)
 	c.ownNet = true
 	c.watchq = opt.Watchq
 	defer func() {