storage controller: issue timeline create/delete calls concurrently (#6827)

## Problem

Timeline creation is meant to be very fast: it should only take
approximately on S3 PUT latency. When we have many shards in a tenant,
we should preserve that responsiveness.

## Summary of changes

- Issue create/delete pageserver API calls concurrently across all >0
shards
- During tenant deletion, delete shard zero last, separately, to avoid
confusing anything using GETs on the timeline.
- Return 201 instead of 200 on creations to make cloud control plane
happy

---------

Co-authored-by: Arpad Müller <arpad-m@users.noreply.github.com>

libs/pageserver_api/src/models.rs

@@ -180,7 +180,7 @@ pub enum TimelineState {
     Broken { reason: String, backtrace: String },
 }
 
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Clone)]
 pub struct TimelineCreateRequest {
     pub new_timeline_id: TimelineId,
     #[serde(default)]