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Fix blocked DB::open on multiprocess access on exFAT filesystem (#6959)

Fix double file lock and DB::open being blocked with multiple concurrent realm access on fat32/exfat file systems.

When file is truncated on fat32/exfat its uid is available for other files. With multiple processes opening and truncating the same set of files could lead to the situation when within one process get_unique_id will return the same value for different files in timing is right. This breaks proper initialization of static data for interprocess mutexes, so that subsequent locks will hang by trying to lock essentially the same file twice.

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