In #9701 there was a change that put the mutex used for `getExpireAdd` directly in `defaultRealCaser`, which is erroneous because multiple filesystems can share the same `caseCache`.
### Purpose
Fixes#9836 and [Slow sync sending files from Android](https://forum.syncthing.net/t/slow-sync-sending-files-from-android/24208?u=marbens). There may be other issues caused by `getExpireAdd` conflicting with itself, though.
### Testing
Unit tests pass and the case cache and conflict detection _seem_ to behave correctly.
Signed-off-by: Marcus B Spencer <marcus@marcusspencer.us>
Marcus B SpencerandGitHub
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