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.

TODO:
- test with user
- regression test

fixes #742