[refactor] new_search api for CancellationToken

[hsqStephenZhang]

Current CancellationToken::new requires a version argument, which is maintained/passed by the frontend.
To make frontend and http api works together, I wonder if we could simplify the version mechanism as the following way:

impl CancellationToken {
    pub fn new_search() -> Self {
        let version = ACTIVE_SEARCH_VERSION.fetch_add(1, Ordering::SeqCst) + 1;
        Self {
            version,
            active_version: &ACTIVE_SEARCH_VERSION,
        }
    }
}

It aligns with the existing new_scan method, which is easier to use.

[ldm0]

Yeah, it's required to make fontend and http api working correctly.

The refactor is okay as long as we track the relationship between frontend's search_version and rust backend's search_version.