storcon: implement safekeeper_migrate handler (#11849)

This PR implements a safekeeper migration algorithm from RFC-035


https://github.com/neondatabase/neon/blob/main/docs/rfcs/035-safekeeper-dynamic-membership-change.md#change-algorithm

- Closes: https://github.com/neondatabase/neon/issues/11823

It is not production-ready yet, but I think it's good enough to commit
and start testing.

There are some known issues which will be addressed in later PRs:
- https://github.com/neondatabase/neon/issues/12186
- https://github.com/neondatabase/neon/issues/12187
- https://github.com/neondatabase/neon/issues/12188
- https://github.com/neondatabase/neon/issues/12189
- https://github.com/neondatabase/neon/issues/12190
- https://github.com/neondatabase/neon/issues/12191
- https://github.com/neondatabase/neon/issues/12192

## Summary of changes
- Implement `tenant_timeline_safekeeper_migrate` handler to drive the
migration
- Add possibility to specify number of safekeepers per timeline in tests
(`timeline_safekeeper_count`)
- Add `term` and `flush_lsn` to `TimelineMembershipSwitchResponse`
- Implement compare-and-swap (CAS) operation over timeline in DB for
updating membership configuration safely.
- Write simple test to verify that migration code works

libs/safekeeper_api/src/models.rs

@@ -210,7 +210,7 @@ pub struct TimelineStatus {
 }
 
 /// Request to switch membership configuration.
-#[derive(Serialize, Deserialize)]
+#[derive(Clone, Serialize, Deserialize)]
 #[serde(transparent)]
 pub struct TimelineMembershipSwitchRequest {
     pub mconf: Configuration,
@@ -221,6 +221,8 @@ pub struct TimelineMembershipSwitchRequest {
 pub struct TimelineMembershipSwitchResponse {
     pub previous_conf: Configuration,
     pub current_conf: Configuration,
+    pub term: Term,
+    pub flush_lsn: Lsn,
 }
 
 #[derive(Clone, Copy, Serialize, Deserialize)]