Always harvest a log that is configured

Discussed in more detail in [googlegroup](https://groups.google.com/forum/#!topic/logstash-users/TwanJlZdho4). I believe logstash-forwarder should always harvest if it exists in the json config. I'm unclear as to why we optimize here and have been burned by the scenario where: 
* log is not rotated every 24hrs
* log was not written to in a 24hr period
* logstash-forwarder is restarted