feature/changelog: Draining of in-transit fops in changelog.

        This is required for Geo-rep to work with snapshots.
        Following things are done in this patch.
        1. Draining of in-transit fops during changelog rollover.
        2. Explicit rollover of changelog when snapshot barrier
           notification comes. During this, intransit fops are
           drained and changelog is rolled over.

        For more details on the purpose of the patch. Please
        visit following link.
        http://www.gluster.org/community/documentation/index.php/Changelog_Design_changes_for_snapshot

Change-Id: I22690131e19d3027f6d8957178bdc3431b9062f6
Signed-off-by: Kotresh H R <khiremat@redhat.com>
Reviewed-on: http://review.gluster.org/7216
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Varun Shastry <vshastry@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>