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.

control_plane/src/pageserver.rs

@@ -419,6 +419,11 @@ impl PageServerNode {
                     .map(|x| x.parse::<bool>())
                     .transpose()
                     .context("Failed to parse 'trace_read_requests' as bool")?,
+                eviction_policy: settings
+                    .get("eviction_policy")
+                    .map(|x| serde_json::from_str(x))
+                    .transpose()
+                    .context("Failed to parse 'eviction_policy' json")?,
             })
             .send()?
             .error_from_body()?;