ORNL-10: Basic IR implementation

ORNL-10: Basic IR implementation

To support imperative recovery, there is a target status table defined for
each file system defined on the MGS. When a target registers itself to the
MGS, the MGS will change this table correspondingly.

In the status table, one important field is target NID. This NID information
is used by clients locating server where target lives. By transferring this NID
to clients, clients can know the restarting of targets earlier. This is
so-called imperative recovery - the MGS notifies clients to do recovery
imperatively instead of timeout based standard recovery.

To implement imperative recovery, clients are asked to cache a NID table, which
contains the location information of all servers. Clients need to hold a read
mode ldlm plain lock - recover lock - to cache this table. Whenever the MGS
wants to change this table, it will enqueue an EXCL recover lock so that all
clients will be notified for this change. Clients will request for a new read
recover lock and then query for the MGS for NID table updates.

Change-Id: I3b38ba142b810df507805b71972feeb1bade1ac2
Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1217
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>