replication: introduce ballot.is_booted

The new field reports whether the instance has finished its
bootstrap/recovery, or IOW has finished box.cfg().

The new field will help in fixing #5613 so as not to try to join
to a replicaset via non-bootstrapped instances if there are
others.

The problem is that otherwise, if all nodes are booted but
are read-only, new instances bootstrap their own independent
replicaset. It would be better to just fail and terminate the
process than do such a bizarre action.

Part of #5613