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.

L’issue non indica file, test o punti di ingresso. Inizia tracciando l’operazione di merge e i tre percorsi di eliminazione concorrenti —pulizia degli snapshot, checkttl ed eliminazione avviata dall’utente— e definisci come completato impedire la NPE segnalata quando i file di merge selezionati vengono eliminati contemporaneamente.

Scritto dal modello di indicizzazione a partire dal testo della issue.