config: don't use an anon replica as an upstream

This commit effectively allows to set `replication.anon: true` without
specifying `replication.peers`.

Without filtering out anonymous replicas from the list of upstreams we
get an error regarding attempt to use an anonymous replica as an
upstream for a non-anonymous instance.

Also, anonymous replicas are excluded from autogenerated upstream list
for other anonymous replicas. It makes the list the same on all the
peers.

A user can configure a custom data flow using `replication.peers`
option.

Part of #9432

NO_DOC=The documentation request is in the last commit of the series.