The PushNotification object is quite big (772 bytes on AMD64),
so avoid doing too many copying without a need.
Collect pointers and do copying only when sending into a channel.
Found using https://go-critic.github.io/overview#rangeValCopy-ref
Linter output:
```
$GOPATH/src/github.com/appleboy/gorush/gorush/worker.go:38:2: rangeValCopy: each iteration copies 772 bytes (consider pointers or indexing)
$GOPATH/src/github.com/appleboy/gorush/gorush/worker.go:53:2: rangeValCopy: each iteration copies 772 bytes (consider pointers or indexing)
```
d275ddbccb