replication: ballot.is_loading -> is_ro

Is_loading in the ballot used to mean the following: "the instance
did not finish its box.cfg() or has read_only = true". Which is
quite a strange property.

For instance, it was 'true' even if the instance is not really
loading anymore but has read_only = true.

The patch renames it to 'is_ro' (which existed here before, but
also with a wrong meaning).

Its behaviour is slightly changed to report the RO state of the
instance. Not its read_only. This way it incorporates all the
possible RO conditions. Such as not finished bootstrap, having
read_only = true, being a Raft follower, and so on.

The patch is done in scope of #5613 where the ballot is going to
be extended and used a bit differently in the join-master search
algorithm.

Part of #5613