Merge concurrency issues with other deleting data operations

After discussion with @zhanglingzhe0820 , there are three possible concurrency problems related to merge
1. For cluster version, after the follower pulls the snapshot file, it will delete the local overlaped file.
2. The checkttl operation will also delete the expired data.
3. The user initiatively initiates the data deletion operation.

Because the merge operation is to select some files to merge, the above operation may also occur when merging, so the merged files may have been deleted, and NPE will appear.

We need to comprehensively consider the concurrency problem to solve all the above problems.

El issue no menciona archivos, pruebas ni puntos de entrada. Empieza rastreando la operación de merge y las tres rutas de eliminación concurrente —limpieza de snapshots, checkttl y eliminación iniciada por el usuario— y define como terminado evitar la NPE reportada cuando se eliminan archivos de merge seleccionados de forma concurrente.

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