[datafusion-cli] Replace mutex with AtomicU64 for stream duration tracking in instrumentedObjectStore

[buraksenn]

Which issue does this PR close?

Related to #18138 but does not close any issue.

Rationale for this change

TimeToFirstItemStream held an Arc<Mutex<Vec>> and a request_index to write back the duration into the shared request list. I saw @alamb and @BlakeOrth's reviews on the PR #19127 about the improvements and wanted to change.

What changes are included in this PR?

• Replace Arc<Mutex<Vec>> + index in TimeToFirstItemStream with a per-request Arc
• Store duration as nanoseconds in AtomicU64 (0 = not yet set) with Release/Acquire ordering
• Start the timer lazily on the first poll_next call instead of at stream creation, so only actual storage latency is measured

Are these changes tested?

Existing tests and I've also added time comparison

Are there any user-facing changes?

No