Remove TimeLogManager and make TimeLogger singleton

[schnellerhase]

Fixes #3480

[garth-wells]

What's the advantage of a monostate design over a singleton? Looks to me to all intents and purposes to be the same thing.

[schnellerhase]

I think in this case it results in the same. We can decide on the interface we prefer, rather calling some kind of an instance function in the singleton approach or the constructor as here with the monostate.

[schnellerhase]

Shall we stick to the monostate or return to the singleton approach, just within TimeLogger now?

[jhale]

After reading the comments I would recommend singleton - as @garth-wells says there is little clear advantage, and singleton is at least well-known (and people understand the issues).