feature/changelog: Virtual xattr to trigger explicit sync in geo-rep.

A virtual xattr "glusterfs.geo-rep.trigger-sync" is provided
in glusterfs through changelog translator. Geo-rep triggers
a explicit data sync on setting this xattr on a file.
Changelog captures a DATA entry on file's gfid on setting this
virtual xattr on a file. This is supported only for files. It
doesn't support directories.

Usage: setfattr -n glusterfs.geo-rep.trigger-sync <file-path>

Change-Id: Ia689326ac2dcb31035ffbecad2c548eda4eb9245
BUG: 1176934
Signed-off-by: Kotresh HR <khiremat@redhat.com>
Reviewed-on: http://review.gluster.org/9337
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Venky Shankar <vshankar@redhat.com>