feat(pageserver): rework reldirv2 rollout (#12576)

## Problem

LKB-197, #9516 

To make sure the migration path is smooth.

The previous plan is to store new relations in new keyspace and old ones
in old keyspace until it gets dropped. This makes the migration path
hard as we can't validate v2 writes and can't rollback. This patch gives
us a more smooth migration path:

- The first time we enable reldirv2 for a tenant, we copy over
everything in the old keyspace to the new one. This might create a short
spike of latency for the create relation operation, but it's oneoff.
- After that, we have identical v1/v2 keyspace and read/write both of
them. We validate reads every time we list the reldirs.
- If we are in `migrating` mode, use v1 as source of truth and log a
warning for failed v2 operations. If we are in `migrated` mode, use v2
as source of truth and error when writes fail.
- One compatibility test uses dataset from the time where we enabled
reldirv2 (of the original rollout plan), which only has relations
written to the v2 keyspace instead of the v1 keyspace. We had to adjust
it accordingly.
- Add `migrated_at` in index_part to indicate the LSN where we did the
initialize.

TODOs:

- Test if relv1 can be read below the migrated_at LSN.
- Move the initialization process to L0 compaction instead of doing it
on the write path.
- Disable relcache in the relv2 test case so that all code path gets
fully tested.

## Summary of changes

- New behavior of reldirv2 migration flags as described above.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

libs/pageserver_api/src/models.rs

@@ -1500,6 +1500,7 @@ pub struct TimelineArchivalConfigRequest {
 #[derive(Serialize, Deserialize, PartialEq, Eq, Clone)]
 pub struct TimelinePatchIndexPartRequest {
     pub rel_size_migration: Option<RelSizeMigration>,
+    pub rel_size_migrated_at: Option<Lsn>,
     pub gc_compaction_last_completed_lsn: Option<Lsn>,
     pub applied_gc_cutoff_lsn: Option<Lsn>,
     #[serde(default)]
@@ -1533,10 +1534,10 @@ pub enum RelSizeMigration {
     /// `None` is the same as `Some(RelSizeMigration::Legacy)`.
     Legacy,
     /// The tenant is migrating to the new rel_size format. Both old and new rel_size format are
-    /// persisted in the index part. The read path will read both formats and merge them.
+    /// persisted in the storage. The read path will read both formats and validate them.
     Migrating,
     /// The tenant has migrated to the new rel_size format. Only the new rel_size format is persisted
-    /// in the index part, and the read path will not read the old format.
+    /// in the storage, and the read path will not read the old format.
     Migrated,
 }
 
@@ -1619,6 +1620,7 @@ pub struct TimelineInfo {
 
     /// The status of the rel_size migration.
     pub rel_size_migration: Option<RelSizeMigration>,
+    pub rel_size_migrated_at: Option<Lsn>,
 
     /// Whether the timeline is invisible in synthetic size calculations.
     pub is_invisible: Option<bool>,