give tantivy a shared thread pool to run fetch doc on

[trinity-1686a]

Description

likely to fix #4940
useless without upgrading tantivy post quickwit-oss/tantivy#2386

[fulmicoton]

This code is correct, but could break if we modified a different part of the code.

It is sometimes acceptable (especially because it is in the same file, and private stuff), but still, I suspect this is unnecessary here.

We could put in the Arc on the stack of the run_cpu_intensive function and extract the ref here (without a &'static lifetime).
Even better I think we can call spawn_blocking directly.

[fulmicoton]

pub async fn run_cpu_intensive<F, R>(cpu_heavy_task: F) -> Result<R, Panicked>
where
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static,
{
    let span = tracing::Span::current();
    search_executor().spawn_blocking(move || {
        let _guard = span.enter();
        let mut active_thread_guard =
            GaugeGuard::from_gauge(&crate::SEARCH_METRICS.active_search_threads_count);
        active_thread_guard.add(1i64);
        cpu_heavy_task()
    }).await.map_err(|_| Panicked)
}

[trinity-1686a]

(marking as wip until it depends on tantivy/main and not an unmerged branch)