features/marker: force xtime updates (configurable) for client-pid = -1

This is required by Geo-Replication that does auxillary mount
with client-pid as -1 (which has special treatment at specific
places in GlusterFS), to trigger xtime updates on the intermediate
master in a cascading setup.

Marker too had a check to "not" mark updates for geo-replication's
auxillary mounts. With the new geo-replication design, xtimes are
not set by the master on the slave for all entities. Due to this
cascading setups were broken.

This patch introduces "geo-replication.ignore-pid-check" option
as a "override" for the client-pid check for gsyncd's client-pid.
When this options is enabled, marker start "marking" even if the
updates are from the special client.

Geo-Replication on the detection of itself being an intermediate
master, enables this option.

Change-Id: I9f7140edd12fef5480595ee0f93f35b94cdb8345
BUG: 996371
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/5591
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Tested-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>