refactor shutdown sequence to use plugin.stop

[jsvd]

Change shutdown sequence to be triggered by plugin.stop instead of ShutdownSignal exception.
Also remove any calls to: shutdown, finished, finished?, running? or terminating?
This depends on elastic/logstash#3895

[untergeek]

Because this plugin very simply calls the tcp input plugin as a super class, this plugin depends on logstash-plugins/logstash-input-tcp#8 being completed.

Once finished, this will work out of the box.

[untergeek]

Pretty sure this is covered now.