s3 WAL offloading staging review.

- Uncomment accidently `self.keep_alive.abort()` commented line, due to this
  task never finished, which blocked launcher.
- Mess up with initialization one more time, to fix offloader trying to back up
  segment 0. Now we initialize all required LSNs in handle_elected,
  where we learn start LSN for the first time.
- Fix blind attempt to provide safekeeper service file with remote storage
  params.

libs/utils/src/zid.rs

@@ -218,7 +218,7 @@ impl ZTenantTimelineId {
 
 impl fmt::Display for ZTenantTimelineId {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}-{}", self.tenant_id, self.timeline_id)
+        write!(f, "{}/{}", self.tenant_id, self.timeline_id)
     }
 }