There may be a theoretical deadlock for the duration of the offset when the system clock is adjusted into a past time while executing SeedStrengthen.
Fix this by using steady clock.
Do the same in FlushStateToDisk, which should make the flushes more steady, if the system clock is adjusted by a large offset.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/27189 | core#27189]]