Push on-demand download into Timeline::get() function itself.

This makes Timeline::get() async, and all functions that call it
directly or indirectly with it. The with_ondemand_download() mechanism
is gone, Timeline::get() now always downloads files, whether you want
it or not. That is what all the current callers want, so even though
this loses the capability to get a page only if it's already in the
pageserver, without downloading, we were not using that capability.
There were some places that used 'no_ondemand_download' in the WAL
ingestion code that would error out if a layer file was not found
locally, but those were dubious. We do actually want to on-demand
download in all of those places.

Per discussion at
https://github.com/neondatabase/neon/pull/3233#issuecomment-1368032358

pageserver/src/http/routes.rs

@@ -13,7 +13,7 @@ use super::models::{
 };
 use crate::pgdatadir_mapping::LsnForTimestamp;
 use crate::tenant::config::TenantConfOpt;
-use crate::tenant::{with_ondemand_download, Timeline};
+use crate::tenant::{PageReconstructError, Timeline};
 use crate::{config::PageServerConf, tenant::mgr};
 use utils::{
     auth::JwtAuth,
@@ -77,6 +77,15 @@ fn check_permission(request: &Request<Body>, tenant_id: Option<TenantId>) -> Res
     })
 }
 
+fn apierror_from_prerror(err: PageReconstructError) -> ApiError {
+    match err {
+        PageReconstructError::Other(err) => ApiError::InternalServerError(err),
+        PageReconstructError::WalRedo(err) => {
+            ApiError::InternalServerError(anyhow::Error::new(err))
+        }
+    }
+}
+
 // Helper function to construct a TimelineInfo struct for a timeline
 async fn build_timeline_info(
     timeline: &Arc<Timeline>,
@@ -298,9 +307,10 @@ async fn get_lsn_by_timestamp_handler(request: Request<Body>) -> Result<Response
         .await
         .and_then(|tenant| tenant.get_timeline(timeline_id, true))
         .map_err(ApiError::NotFound)?;
-    let result = with_ondemand_download(|| timeline.find_lsn_for_timestamp(timestamp_pg))
+    let result = timeline
+        .find_lsn_for_timestamp(timestamp_pg)
         .await
-        .map_err(ApiError::InternalServerError)?;
+        .map_err(apierror_from_prerror)?;
 
     let result = match result {
         LsnForTimestamp::Present(lsn) => format!("{lsn}"),