replication: remove unnecessary errors on replicating from an anonymous instance

Since the anonymous replica implementation, it was forbidden to
replicate (join/subscribe/register) from anonymous instances.
Actually, only joining and register should be banned, since an anonymous
replica isn't able to register its peer in _cluster anyway.

Let's allow other anonymous replicas, but not the normal ones, to subscribe to
an anonymous replica.
Also remove unnecessary ER_UNSUPPORTED errors from box_process_join()
and box_process_register() for anonymous replicas. These cases are
covered with ER_READONLY checks later on, since anonymous replicas must
be read-only.

Note, this patch doesn't allow normal instances to subscribe to
anonymous ones. Even though it is technically possible, it may bring
more problems than profits. Let's allow it later on if there's an explicit
demand.

Closes #4696