automatic layer eviction

This patch adds a per-timeline periodic task that executes an eviction
policy. The eviction policy is configurable per tenant.

Two policies exist:
- NoEviction (the default one)
- LayerAccessThreshold

The LayerAccessThreshold policy examines the last access timestamp per
layer in the layer map and evicts the layer if that last access is
further in the past than a configurable threshold value.
This policy kind is evaluated periodically at a configurable period.
It logs a summary statistic at `info!()` or `warn!()` level, depending
on whether any evictions failed.

This feature has no explicit killswitch since it's off by default.

pageserver/src/http/routes.rs

@@ -797,6 +797,14 @@ async fn update_tenant_config_handler(
         );
     }
 
+    if let Some(eviction_policy) = request_data.eviction_policy {
+        tenant_conf.eviction_policy = Some(
+            serde_json::from_value(eviction_policy)
+                .context("parse field `eviction_policy`")
+                .map_err(ApiError::BadRequest)?,
+        );
+    }
+
     let state = get_state(&request);
     mgr::set_new_tenant_config(state.conf, tenant_conf, tenant_id)
         .instrument(info_span!("tenant_config", tenant = ?tenant_id))