LU-1520 ldlm: improve ldlm_pools_shrink algorithm

LU-1520 ldlm: improve ldlm_pools_shrink algorithm

1, shrink namespaces by batches of 64 namespaces, the batch is
   implemented as list
2, limit number of simultaneously shrinking threads to 32 threads
3, have ldlm_pools_recalc to operate with namespaces similar to
   ldlm_pools_shrink
4, use glboal counters of unused locks on client and granted
   locks on servers to avoid iterating over namespaces

Change-Id: I5fb3f56748ae10961c50b4b06c300c9c7f5fca87
Signed-off-by: Vladimir Saveliev <valdimir.saveliev@oracle.com>
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>