geo-rep/libgfchangelog: Create working dir during changelog_register if not present.

Earlier, xysnc's register was being called first,
which was creating working directory before calling
changelog_register. Now it is history crawl first.
Hence working directory would not have been created.
Create it in gf_changelog_register itself if it is
not already created.

Change-Id: Ief3f2b87deaf5da16c135b64be1be42e0a7647f3
BUG: 1125843
Signed-off-by: Kotresh H R <khiremat@redhat.com>
Reviewed-on: http://review.gluster.org/8399
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Aravinda VK <avishwan@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>