The Apache Mina project recommends NIO2; since If047c3a2 the sshd MINA backend is partly broken as well, one more reason to abandon it as default value. Using a more stable and recommended backend provides a better out-of-the-box experience for the new Gerrit installations. Change-Id: I48c69dd3e6c9c05afe5da35a84c9b2de826e01e6