fix: replication of placeholder filesystems (#744)

fixes https://github.com/zrepl/zrepl/issues/742

Before this PR, when chaining replication from
A => B => C, if B had placeholders and the `filesystems`
included these placeholders, we'd incorrectly
fail the planning phase with error
`sender does not have any versions`.

The non-placeholder child filesystems of these placeholders
would then fail to replicate because of the
initial-replication-dependency-tracking that we do, i.e.,
their parent failed to initially replication, hence
they fail to replicate as well
(`parent(s) failed during initial replication`).

We can do better than that because we have the information
whether a sender-side filesystem is a placeholder.
This PR makes the planner act on that information.
The outcome is that placeholders are replicated as
placeholders (albeit the receiver remains in control
of how these placeholders are created, i.e., `recv.placeholders`)
The mechanism to do it is:
1. Don't plan any replication steps for filesystems that
   are placeholders on the sender.
2. Ensure that, if a receiving-side filesystem exists, it
   is indeed a placeholder.

Check (2) may seem overly restrictive, but, the goal here
is not just to mirror all non-placeholder filesystems, but
also to mirror the hierarchy.

Testing performed:
- [x] confirm with issue reporter that this PR fixes their issue
- [x] add a regression test that fails without the changes in this PR

replication/logic/replication_logic.go

@@ -333,6 +333,22 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 
 	log(ctx).Debug("assessing filesystem")
 
+	if fs.senderFS.IsPlaceholder {
+		log(ctx).Debug("sender filesystem is placeholder")
+		if fs.receiverFS != nil {
+			if fs.receiverFS.IsPlaceholder {
+				// all good, fall through
+				log(ctx).Debug("receiver filesystem is placeholder")
+			} else {
+				err := fmt.Errorf("sender filesystem is placeholder, but receiver filesystem is not")
+				log(ctx).Error(err.Error())
+				return nil, err
+			}
+		}
+		log(ctx).Debug("no steps required for replicating placeholders, the endpoint.Receiver will create a placeholder when we receive the first non-placeholder child filesystem")
+		return nil, nil
+	}
+
 	sfsvsres, err := fs.sender.ListFilesystemVersions(ctx, &pdu.ListFilesystemVersionsReq{Filesystem: fs.Path})
 	if err != nil {
 		log(ctx).WithError(err).Error("cannot get remote filesystem versions")