close connection on read error

otherwise it will just keep trying forever, and stuff like this happens:

[root@statsd log]# uniq  -c statsdaemon.log
      1 2014/03/04 23:55:54 statsdaemon instance 'gce-central' starting
      1 2014/03/04 23:55:54 listening on :8125
      1 2014/03/04 23:55:54 listening on :8124
      1 Listening on :8126
187967132 Error reading: use of closed network connection
      1 !! Caught signal 15... shutting down
gofmt
bugfix: still send internal metrics even if there were no external metrics
debug when submit() finishes