Add parallel file deletion support for ExpireSnapshots cleanup

[slfan1989]

Background

ExpireSnapshots may delete many files during cleanup, including data files, manifest files, manifest list files, and statistics files.

Currently, ExpireSnapshots already uses FileIO ::DeleteFiles for grouped deletion. However, the cleanup path itself still processes deletion groups sequentially. For large tables with many expired files, this may make snapshot expiration cleanup slower than necessary.

Proposal

Add parallel file deletion support inside the ExpireSnapshots cleanup strategy.

The initial implementation can be intentionally scoped and conservative:

• Keep the public ExpireSnapshots API unchanged.
• Use an internal default parallelism value.
• Apply parallelism only inside the cleanup deletion path.
• Preserve existing best-effort cleanup semantics.
• Continue supporting custom DeleteWith(...) callbacks.