LU-1396 osc: control the RPC rate between MDS and OST

LU-1396 osc: control the RPC rate between MDS and OST

1. Limit the RPC rate with setting 50 maxim rpc in flight between
MDS and OST.

2. Add specify flag in oa to tell whether the destory from echo_md
destory or MDT close/unlink orphan, where we can not throttle the
destory RPC, since it might block ptlrpcd thread. See Bug 16006.

Signed-off-by: Wang Di <di.wang@whamcloud.com>
Change-Id: Ie3b555eecd2716a82b058f645d002f46d8c4dd36
Reviewed-on: http://review.whamcloud.com/2899
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>