Merge #10231: [Qt] Reduce a significant cs_main lock freeze

Summary:
4082fb0 Add missing <atomic> header in clientmodel.h (Jonas Schnelli)
928d4a9 Set both time/height header caches at the same time (Jonas Schnelli)
610a917 Declare headers height/time cache mutable, re-set the methods const (Jonas Schnelli)
cf92bce Update the remaining blocks left in modaloverlay at init. (Jonas Schnelli)
7148f5e Reduce cs_main locks during modal overlay by adding an atomic cache (Jonas Schnelli)

Tree-SHA512: a92ca22f90b8b2a5e8eb94fdce531ef44542e21a8dbbb0693f7723d7018592cb68de687a2a0aac91d31cbf019793f8e922550656d2b130ed3d854d60630341db

Backport of Core PR10231
https://github.com/bitcoin/bitcoin/pull/10231/files
Completes T510

Test Plan:
make check
Run bitcoin-qt for sanity

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2429