LU-4687 obdclass: unified flow control interfaces

LU-4687 obdclass: unified flow control interfaces

Unify the flow control interfaces for MDC RPC, FLD RPC and lfsck
remote update (via OSP). We allow to adjust the maximum inflight
RPCs count via /proc interface.

Rename some variables/functions to make them easily understood.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I096a1de17fe226bac2fe3a3c891365c2e2c51ef3
Reviewed-on: http://review.whamcloud.com/9562
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>