LU-5319 mdc: add max modify RPCs in flight variable

LU-5319 mdc: add max modify RPCs in flight variable

This patch introduces the maximum modify RPCs in flight variable of
a mdc client obd device. Its value is set from connection flag and
and connection data. It can later be tuned through the
max_mod_rpcs_in_flight procfs file.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: I51600f151fd508450d1898647a94856f16d2b245
Reviewed-on: http://review.whamcloud.com/14153
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>