Allow configuration of SSH rekey values

A long standing bug in SSHD Mina where packets gets out of
order during the rekey phase, will force the ControlMaster
to disconnect during high load (repo -jxx):

  Protocol error: expected packet type 31, got 94

This fix allows us to change the trigger limits for rekeying:

  rekeyTimeLimit - Default value 1h
  rekeyBytesLimit - Default value 1073741824 (bytes, 1GB)

The rekeyTimeLimit can be set to 0 to disable the trigger.

The rekeyBytesLimit can never be set to lower than 32, (Mina
will automatically raise the value to 32 if lower).

To get rid of the Protocol error by using these configuration
values, you can set rekeyTimeLimit to 0 (disables it) and also
set a very high rekeyBytesLimit. This will ensure that the bug
prone rekeying logic in SSHD Mina will never be executed.

Change-Id: Ic196c99266252f47659b26af645b26f77fc15ebd

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

@@ -189,6 +189,15 @@ public class SshDaemon extends SshServer implements SshInfo, LifecycleListener {
         IDLE_TIMEOUT,
         String.valueOf(SECONDS.toMillis(idleTimeoutSeconds)));
 
+    long rekeyTimeLimit = ConfigUtil.getTimeUnit(cfg, "sshd", null,
+        "rekeyTimeLimit", 3600, SECONDS);
+    getProperties().put(
+        REKEY_TIME_LIMIT,
+        String.valueOf(SECONDS.toMillis(rekeyTimeLimit)));
+
+    getProperties().put(REKEY_BYTES_LIMIT,
+        String.valueOf(cfg.getLong("sshd", "rekeyBytesLimit", 1024 * 1024 * 1024 /* 1GB */)));
+
     final int maxConnectionsPerUser =
         cfg.getInt("sshd", "maxConnectionsPerUser", 64);
     if (0 < maxConnectionsPerUser) {