LU-6003 lnet: improvement to router checker

This patch starts router checker thread all the time.

The router checker only checks routes by ping if
live_router_check_interval or dead_router_check_interval are set
to something other than 0, and there are routes configured.

If these conditions are not met the router checker sleeps until woken
up when a route is added.  It is also woken up whenever the RC is
being stopped to ensure the thread doesn't hang.

In the future when DLC starts configuring the live and dead
router_check_interval parameters, then by manipulating them
the router checker can be turned on and off by the user.

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: I778690755e7121abd575f1a261637cb6dc754edd
Reviewed-on: http://review.whamcloud.com/13035
Tested-by: Jenkins
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>