Add shut-down logic for subscriber/publisher

[alexellis]

Expected Behaviour

The subscriber (queue-worker) and publisher (gateway) should not try to reconnect if the connection is being broken due to SIGTERM/SIGINT.

Current Behaviour

Once #50 is merged they should both try to re-try during the period at which they are terminating.

Possible Solution

The watchdog implements similar logic, it could probably be adapted for use here: https://github.com/openfaas/faas/blob/master/watchdog/main.go#L86

[bartsmykla]

Derek assign: me

[bartsmykla]

I want to do that. It should be quick thing

[embano1]

@bartsmykla has this been resolved with your PR so this issue can be closed? Thx for your work!