pageserver: add wait_until_flushed parameter for timeline checkpoint (#10013)

## Problem

I'm writing an ingest benchmark in #9812. To time S3 uploads, I need to
schedule a flush of the Pageserver's in-memory layer, but don't actually
want to wait around for it to complete (which will take a minute).

## Summary of changes

Add a parameter `wait_until_flush` (default `true`) for
`timeline/checkpoint` to control whether to wait for the flush to
complete.

pageserver/src/http/routes.rs

@@ -2148,16 +2148,20 @@ async fn timeline_checkpoint_handler(
     // By default, checkpoints come with a compaction, but this may be optionally disabled by tests that just want to flush + upload.
     let compact = parse_query_param::<_, bool>(&request, "compact")?.unwrap_or(true);
 
+    let wait_until_flushed: bool =
+        parse_query_param(&request, "wait_until_flushed")?.unwrap_or(true);
+
     let wait_until_uploaded =
         parse_query_param::<_, bool>(&request, "wait_until_uploaded")?.unwrap_or(false);
 
     async {
         let ctx = RequestContext::new(TaskKind::MgmtRequest, DownloadBehavior::Download);
         let timeline = active_timeline_of_active_tenant(&state.tenant_manager, tenant_shard_id, timeline_id).await?;
-        timeline
-            .freeze_and_flush()
-            .await
-            .map_err(|e| {
+        if wait_until_flushed {
+            timeline.freeze_and_flush().await
+        } else {
+            timeline.freeze().await.and(Ok(()))
+        }.map_err(|e| {
                 match e {
                     tenant::timeline::FlushLayerError::Cancelled => ApiError::ShuttingDown,
                     other => ApiError::InternalServerError(other.into()),