manager: rework the timestamps logic, so that they are an enum-index array

manager: rework the timestamps logic, so that they are an enum-index array

This makes things quite a bit more systematic I think, as we can
systematically operate on all timestamps, for example for the purpose of
serialization/deserialization.

This rework doesn't necessarily make things shorter in the individual
lines, but it does reduce the line count a bit.

(This is useful particularly when we want to add additional timestamps,
for example to solve #7023)