replication: prefer to join from booted replicas

The algorithm of looking for an instance to join the replicaset
from didn't take into account that some of the instances might be
not bootstrapped but still perfectly available.

As a result, a ridiculous situation could happen - an instance
could connect to a cluster with just read-only instances, but it
could have itself with box.cfg{read_only = false}. Then instead of
failing or waiting it just booted a brand new cluster. And after
that the node just started complaining about the others having a
different replicaset UUID.

The patch makes so a new instance always prefers a bootstrapped
join-source to a non-boostrapped one, including self. In the
situation above the new instance now terminates with an error.

In future hopefully it should start a retry-loop instead.

Closes #5613

@TarantoolBot document
Title: IPROTO_BALLOT rework and a new field

A couple of fields in `IPROTO_BALLOT 0x29` used to have values not
matching with their names. They are changed.

* `IPROTO_BALLOT_IS_RO 0x01` used to mean "the instance has
  `box.cfg{read_only = true}`". It was renamed in the source code
  to `IPROTO_BALLOT_IS_RO_CFG`. It has the same code `0x01`, and
  the value is the same. Only the name has changed, and in the doc
  should be too.

* `IPROTO_BALLOT_IS_LOADING 0x04` used to mean "the instance has
  finished `box.cfg()` and it has `read_only = true`". The name
  was wrong therefore, because even if the instance finished
  loading, the flag still was false for `read_only = true` nodes.
  Also such a value is not very suitable for any sane usage.
  The name was changed to `IPROTO_BALLOT_IS_RO`, the code stayed
  the same, and the value now is "the instance is not writable".
  The reason for being not writable can be any: the node is an
  orphan; or it has `read_only = true`; or it is a Raft follower;
  or anything else.

And there is a new field.

`IPROTO_BALLOT_IS_BOOTED 0x06` means the instance has finished its
bootstrap or recovery.