maintain access stats for historic layers

This patch adds basic access statistics for historic layers
and exposes them in the management API's `LayerMapInfo`.

We record the accesses in the `{Delta,Image}Layer::load()` function
because it's the common path of
* page_service (`Timline::get_reconstruct_data()`)
* Compaction (`PersistentLayer::iter()` and `PersistentLayer::key_iter()`)

The stats survive residence status changes, and record these as well.

When scraping the layer map endpoint to record its evolution over time,
one must account for stat resets because they are in-memory only and
will reset on pageserver restart.
Use the launch timestamp header added by (#3527) to identify pageserver restarts.

This is PR https://github.com/neondatabase/neon/pull/3496

pageserver/src/task_mgr.rs

@@ -169,7 +169,14 @@ task_local! {
 /// Note that we don't try to limit how many task of a certain kind can be running
 /// at the same time.
 ///
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[derive(
+    Debug,
+    // NB: enumset::EnumSetType derives PartialEq, Eq, Clone, Copy
+    enumset::EnumSetType,
+    serde::Serialize,
+    serde::Deserialize,
+    strum_macros::IntoStaticStr,
+)]
 pub enum TaskKind {
     // Pageserver startup, i.e., `main`
     Startup,