Wait for secondary state after stepdown command in multi_rs.js

SERVER-31265 fixes the race of closing connection for new connections in 3.6 and beyond, but wasn't backported to 3.4. That causes the new connection for "ismaster" to be closed after sending stepdown command in multi_rs.js on 3.4. A workaround is to wait for "secondary" state after sending stepdown command but before calling "reconnect" which sends an "ismaster".