Feed instance abrupt termination when unable to de-register on a subset of attached NLB target groups

[Supreeth Rao]

Same Feed instance can attach to multiple target groups. During shutdown, it will then attempt to de-register itself from all of the target groups. In case of a failure to de-register from any of them, feed instance will not honour the shutdown or the drain interval and shutdown immediately hence causing an ungraceful shutdown for the valid registered target groups

https://github.com/sky-uk/feed/blob/master/nlb/nlb.go#L348-L359