storcon: automatically clear Pause/Stop scheduling policies to enable detaches (#10011)

## Problem

We saw a tenant get stuck when it had been put into Pause scheduling
mode to pin it to a pageserver, then it was left idle for a while and
the control plane tried to detach it.

Close: https://github.com/neondatabase/neon/issues/9957

## Summary of changes

- When changing policy to Detached or Secondary, set the scheduling
policy to Active.
- Add a test that exercises this
- When persisting tenant shards, set their `generation_pageserver` to
null if the placement policy is not Attached (this enables consistency
checks to work, and avoids leaving state in the DB that could be
confusing/misleading in future)

libs/pageserver_api/src/controller_api.rs

@@ -245,6 +245,17 @@ impl From<NodeAvailability> for NodeAvailabilityWrapper {
     }
 }
 
+/// Scheduling policy enables us to selectively disable some automatic actions that the
+/// controller performs on a tenant shard. This is only set to a non-default value by
+/// human intervention, and it is reset to the default value (Active) when the tenant's
+/// placement policy is modified away from Attached.
+///
+/// The typical use of a non-Active scheduling policy is one of:
+/// - Pinnning a shard to a node (i.e. migrating it there & setting a non-Active scheduling policy)
+/// - Working around a bug (e.g. if something is flapping and we need to stop it until the bug is fixed)
+///
+/// If you're not sure which policy to use to pin a shard to its current location, you probably
+/// want Pause.
 #[derive(Serialize, Deserialize, Clone, Copy, Eq, PartialEq, Debug)]
 pub enum ShardSchedulingPolicy {
     // Normal mode: the tenant's scheduled locations may be updated at will, including