replication: allow to re-register with new UUID

Previously it wasn't allowed to change instance UUID in _cluster.
When needed, it had to be done manually by deleting the instance
from _cluster and inserting it back with a new UUID. Or not to be
done at all.

Re-UUID (like re-name) was reported to be used when people didn't
want to register new replica IDs. They wanted to rejoin lost
replicas from scratch but keep the numeric ID. With UUID they
could deal by either setting it explicitly to the old value on a
new instance, or by doing the manual re-UUID like described above.

This commit is supposed to make things simpler. If a replica has a
name, then its re-join with another UUID is not an error. Its
record in _cluster is automatically updated to store the new UUID.

That is only possible if the old-UUID-instance is not connected
anymore and is not listed in replication cfg.

Closes #5029

@TarantoolBot document
Title: Instance rebootstrap with new UUID but same ID and name
If an instance has a non-empty instance name
(`box.cfg.instance_name`), then at rebootstrap it can keep the
name and its old numeric ID (space `_cluster['id']` field).

This might be needed if one doesn't want to pollute `_cluster`
with new rows, and somewhy doesn't want to or can't just drop the
rows belonging to the dead replicas.

In order for this to work 1) the rebootstrapping replica must keep
its old non-empty instance name, 2) the other instances should not
have any alive connections to the old dead replica. Ideally, the
old replica should be just deleted from `box.cfg.replication`
everywhere.

When that works, the old row in `_cluster` is automatically
updated with the new instance UUID.