mw/health: call Shutdown on FinalShutdown (#1003)
mw/health: call Shutdown on FinalShutdown (#1003)

Reloading caddy won't kill the health handler. Only on final shutdown
we stop the handler.

Currently when reloading CoreDNS with -SIGUSR1 the health handler stops
answering - there is a test for this but it doesn't capture whole
process reloading, sadly. This PR keeps the handler alive during reloads
and only stops on process shutdown.
coredns/coredns,johnbelamaric/coredns,miekg/coredns,jremond/coredns,miekg/coredns,jremond/coredns,ii/coredns,johnbelamaric/coredns,chrisohaver/coredns,yongtang/coredns,ii/coredns