Speed up layer_map::search, by remembering the "envelope" for each layer.

Lookups in the R-tree call the "envelope" function for every comparison,
and our envelope function isn't very cheap, so that overhead adds up.
Create the envelope once, when the layer is inserted into the tree, and
store it along with the layer. That uses some more memory per layer, but
that's not very significant.

Speeds up the search operation 2x

pageserver/src/tenant/layer_map.rs

@@ -62,6 +62,8 @@ pub struct LayerMap {
 
 struct LayerRTreeObject {
     layer: Arc<dyn Layer>,
+
+    envelope: AABB<[IntKey; 2]>,
 }
 
 // Representation of Key as numeric type.
@@ -197,9 +199,16 @@ impl PartialEq for LayerRTreeObject {
 impl RTreeObject for LayerRTreeObject {
     type Envelope = AABB<[IntKey; 2]>;
     fn envelope(&self) -> Self::Envelope {
-        let key_range = self.layer.get_key_range();
-        let lsn_range = self.layer.get_lsn_range();
-        AABB::from_corners(
+        self.envelope
+    }
+}
+
+impl LayerRTreeObject {
+    fn new(layer: Arc<dyn Layer>) -> Self {
+        let key_range = layer.get_key_range();
+        let lsn_range = layer.get_lsn_range();
+
+        let envelope = AABB::from_corners(
             [
                 IntKey::from(key_range.start.to_i128()),
                 IntKey::from(lsn_range.start.0 as i128),
@@ -208,7 +217,8 @@ impl RTreeObject for LayerRTreeObject {
                 IntKey::from(key_range.end.to_i128() - 1),
                 IntKey::from(lsn_range.end.0 as i128 - 1),
             ], // AABB::upper is inclusive, while `key_range.end` and `lsn_range.end` are exclusive
-        )
+        );
+        LayerRTreeObject { layer, envelope }
     }
 }
 
@@ -338,7 +348,7 @@ impl LayerMap {
         if layer.get_key_range() == (Key::MIN..Key::MAX) {
             self.l0_delta_layers.push(layer.clone());
         }
-        self.historic_layers.insert(LayerRTreeObject { layer });
+        self.historic_layers.insert(LayerRTreeObject::new(layer));
         NUM_ONDISK_LAYERS.inc();
     }
 
@@ -362,7 +372,7 @@ impl LayerMap {
         }
         assert!(self
             .historic_layers
-            .remove(&LayerRTreeObject { layer })
+            .remove(&LayerRTreeObject::new(layer))
             .is_some());
         NUM_ONDISK_LAYERS.dec();
     }