CBD-1425:  only download manifest if it's current.xml

Change-Id: Ie91428b1ed9c6c8edae816c811e8b57d4baaa40d
Reviewed-on: http://review.couchbase.org/40423
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>