Fix problem with graphite talking to closed connections (#2171)

We were having problems with telegraf talking to
carbon-relay-ng using the graphite output. When
the carbon-relay-ng server restarted the connection
the telegraf side would go into CLOSE_WAIT but telegraf
would continue to send statistics through the connection.

Reading around it seems you need to a read from the connection
and see a EOF error. We've implemented this and added a test
that replicates roughly the error we were having.

Pair: @whpearson @joshmyers