Switch to glog for logging

1) imported glog to third_party (previous commit)
2) add support for third_party/update.sh to update just one pkg
3) search-and-replace:
  s/log.Printf/glog.Infof/
  s/log.Print/glog.Info/
  s/log.Fatalf/glog.Fatalf/
  s/log.Fatal/glog.Fatal/
4) convert glog.Info.*, err into glog.Error*

Sadly, we have to keep passing etcd a log.Logger - it is not defined as an
interface so glog can not implement it.  Sigh.  Maybe we can convince etcd
code to define a minimal interface that both log.Logger and glog can
implement.  That's for another day.