Parallelize async MonitorUpdatingPersister reads

[TheBlueMatt]

Now that MonitorUpdatingPersister has an async mode in #4063, we should take advantage of that on the read side to parallelize things, probably making a huge impact on startup time.

[Abeeujah]

Hi @TheBlueMatt is this issue stale?

[TheBlueMatt]

Oops, no, I think this was done in #4147 and to a lesser extent #4266. If you spot any other parts of init that could be better parallelized, please share!