PR description:
> During the GUI startup, there is currently an accumulation of cs_main locks due to setting initial chain state values at multiple locations (in the GUI main thread).
>
> This PR tries to cache the initial chain state (tip height, tip time, best header, etc.) short after loading the blockindex.
>
> The cached values are then used instead of fetching them again (and thus locking cs_main) during setting the client model.
>
> This should fix the initial GUI blocking often experienced during or short after the splashscreen.
This concludes backport of core#19011 [4/4]
https://github.com/bitcoin/bitcoin/pull/19011/commits/386ec192a57b76492125d691ceda1b4aa832312e
Depends on D9735