LU-7236 ptlrpc: idle connections can disconnect

LU-7236 ptlrpc: idle connections can disconnect

 - when new request is being allocated ptlrpc initiates
   connection if it's not connected yet
 - if the import is idle (no locks, no active RPCs, no
   non-PING reply for last osc_idle_timeout seconds),
   then pinger tries to disconnect asynchronously
 - currently only client-to-OST connections can be idle
 - lctl set_param osc.*.idle_timeout=N controls new feature:
   N=0 - disable
   N>0 - seconds to idle before disconnect
 - lctl set_param osc.*.idle_connect=N to reconnect if idle
   (N is positive number)
 - OSC module parameter osc_idle_timeout controls default
   idle timeout and set to 20 seconds by default

Change-Id: I4b90eb5209a0b0e62d85fd55ad6e9cab8c03fd14
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/16682
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>