manager: introduce MANAGER_IS_FINISHED() macro

manager: introduce MANAGER_IS_FINISHED() macro

Let's make our finished checks a bit more readable. Checking the
timestamp is not entirely obvious, hence let's abstract that a bit by
adding a macro that shows what we are doing here, not how we doing it.

This is particularly useful if we want to change the definition of
"finished" later on, in particular, when we try to fix #7023.