Adds logical deletes to shared channel remotes and remote clusters (#28159)

* Adds logical deletes to shared channel remotes and remote clusters

Instead of physically deleting the shared channel remote and remote
clusters records when a channel is unshared, a remote uninvited or a
remote cluster is deleted, now those have a logical `DeleteAt` field
that is set.

This allows us to safely restore shared channels between two remote
clusters (as of now resetting the cursor without backfilling their
contents) and to know which connections were established in the past
and now are severed.

* Delete the index in remoteclusters before adding the new column

* Fix bad error check

server/platform/services/sharedchannel/sync_send.go

@@ -343,6 +343,9 @@ func (scs *Service) processTask(task syncTask) error {
 		if err != nil {
 			return err
 		}
+		if rc.DeleteAt != 0 {
+			return fmt.Errorf("Processing task for a deleted remote cluster '%s'", task.remoteID)
+		}
 		if !rc.IsOnline() {
 			return fmt.Errorf("Failed updating shared channel '%s' for offline remote cluster '%s'", task.channelID, rc.DisplayName)
 		}