replication: fix bug with read-only replica as a bootstrap leader

Another broken case. Adding a new replica to cluster:
+		if (replica->applier->remote_is_ro &&
+		    replica->applier->vclock.signature == 0)
In this case we may got an ER_READONLY, since signature is not 0.
So leader election now has two phases:
 1. To select among read-write replicas.
 2. If no such found, try old algorithm for backward compatibility
    (case then all replicas exist in cluster table).

Closes #3257