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.

The issue names no files, tests, or entry points. Start by tracing the merge operation and the three concurrent deletion paths—snapshot cleanup, checkttl, and user-initiated deletion—and define done as preventing the reported NPE when selected merge files are deleted concurrently.

Written by the indexing model from the issue text.