Multiplatform synchronization (Use kotlinx-atomicfu)

[westnordost]

synchronized is a JVM-only API in Kotlin and as far as we know, there are no plans to change that.

synchronized is used in StreetComplete in many places in the data layer. The data layer is architecture-wise a blocking API (i.e. no coroutines, no suspending functions) and hence we can't use Mutex for threadsafe locks. (Let's not preclude that we refactor the data layer one day to be coroutine-based although it would be a considerable effort, but for now:)

We need a multiplatform replacement for synchronized. The most obvious choice is the official kotlinx-atomicfu, which is however still marked as experimental, especially the locks.
So, while there are other things to do in preparation of #5412, it might be better to hold off towards (at least) merging a PR that implements this (because according to one maintainer of kotlinx-atomicfu, they are working towards stabilization)