pageserver: hold GC while reading from a timeline (neondatabase#10559)

## Problem

If we are GC-ing because a new image layer was added while traversing
the timeline, then it will remove layers that are required for
fulfilling the current get request (read-path cannot "look back" and
notice the new image layer).

## Summary of Changes

Prevent GC from progressing on the current timeline while it is being
visited for a read.

Epic: neondatabase#9376

pageserver/src/tenant/timeline.rs

@@ -3468,6 +3468,13 @@ impl Timeline {
         let mut completed_keyspace = KeySpace::default();
         let mut image_covered_keyspace = KeySpaceRandomAccum::new();
 
+        // Prevent GC from progressing while visiting the current timeline.
+        // If we are GC-ing because a new image layer was added while traversing
+        // the timeline, then it will remove layers that are required for fulfilling
+        // the current get request (read-path cannot "look back" and notice the new
+        // image layer).
+        let _gc_cutoff_holder = timeline.get_latest_gc_cutoff_lsn();
+
         loop {
             if cancel.is_cancelled() {
                 return Err(GetVectoredError::Cancelled);