Set default sshd backend to NIO2

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

Documentation/config-gerrit.txt

@@ -3452,7 +3452,7 @@ Starting from version 0.9.0 Apache SSHD project added support for NIO2
 IoSession. To use the new NIO2 session the `backend` option must be set
 to `NIO2`.
 +
-By default, `MINA`.
+By default, `NIO2`.
 
 [[sshd.listenAddress]]sshd.listenAddress::
 +

gerrit-sshd/src/main/java/com/google/gerrit/sshd/SshDaemon.java

@@ -211,7 +211,7 @@ public class SshDaemon extends SshServer implements SshInfo, LifecycleListener {
         "sshd", "enableCompression", false);
 
     SshSessionBackend backend = cfg.getEnum(
-        "sshd", null, "backend", SshSessionBackend.MINA);
+        "sshd", null, "backend", SshSessionBackend.NIO2);
 
     System.setProperty(IoServiceFactoryFactory.class.getName(),
         backend == SshSessionBackend.MINA