Service interface: add GetExitChan function

[mdh67899]

see nsqio/nsq#1140, thanks。

[judwhite]

@mdh67899 I believe this is addressed with the addition of the Context interface (thanks @jehiah)

go-svc/svc.go

Lines 49 to 54 in 054d56d

	// Context interface contains an optional Context function which a Service can implement.
	// When implemented the context.Done() channel will be used in addition to signal handling
	// to exit a process.
	type Context interface {
	Context() context.Context
	}

Example:

go-svc/example/main.go

Lines 14 to 31 in 054d56d

	type program struct {
	LogFile *os.File
	svr *server
	ctx context.Context
	}
	func (p *program) Context() context.Context {
	return p.ctx
	}
	func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Minute)
	defer cancel()
	prg := program{
	svr: &server{},
	ctx: ctx,
	}