+1 for a feature like this.
Where can I promote such a feature request?
There is no reason, an app I use only sometimes should run all the time. Saves memory and battery.
You can also disable messenger apps is you want to be offline and don't get new messages.
I used Apps like AirFrozen and other on rooted devices, but I think it would be more secure if this is a feature of the OS and not a third party app.
The method via "Settings -> Apps" works, but its not that convenient.