LU-2924 ldlm: split client namespaces into active and inactive

The main reason behind this is ldlm_poold walks all namespaces currently
no matter if there are any locks or not. On large systems this could take
quite a bit of time, esp. since ldlm_poold is currently woken up once per
second.

Now every time a client namespace loses it's last resource it is placed
into an inactive list that is not touched by ldlm_poold as pointless.
On creation of a first resource in a namespace it is placed back into
the active list.

Change-Id: I15d9b731d922e073eb9e273c3e19e84cab377916
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/5624
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Hiroya Nozaki <nozaki.hiroya@jp.fujitsu.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>