When old master restores, it should be marked as unhealthy
Related to #2571 (closed)
After !1800 (merged)
Few times I've hit a case where an old master returning from being down still gets marked as master. This leads to having multiple masters and causes issues. Good thing is that we automatically stop pgbouncer from running when we see more than one master so rule out any damage. The bad thing is that this basically causes downtime. We should look whether we can mark an old master unhealthy when it returns to rotation.