[datafusion-cli] Replace mutex with AtomicU64 for stream duration tracking in instrumentedObjectStore (apache#20802)

## Which issue does this PR close?
Related to apache#18138 but does not close any issue.

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

## What changes are included in this PR?
- Replace Arc<Mutex<Vec<RequestDetails>>> + index in
TimeToFirstItemStream with a per-request Arc<AtomicU64>
- 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

Author: buraksenn