add `add_timer` functionality

[pohmelie]

For fast prototyping Service.timer decorator is enough, but for flexible solution this need to be improved because this do not allows to:

• Set timer interval at runtime (from config, cli, etc.)
• Stop timer
• Add timer at runtime

I think best approach here is pretty similar to others add_* methods. Something like add_timer will create new timer and return handle to manipulate (stop) it.