cluster/dht: Del GF_READDIR_SKIP_DIRS key from dict for first_up

Currently, we sent GF_READDIR_SKIP_DIRS for all subvolumes if
first_subvol != first_up_subvolume.

Also first_up_subvolume can change with-in the life of a call and
cbk. Saving the first_up_subvol in dht_local for checks.

Back porting fix http://review.gluster.org/5577

BUG: 996474
Change-Id: I67b5bbe781e12812557b569b7d0a0beba4224159
Signed-off-by: shishir gowda <gowda.shishir@gmail.com>
Reviewed-on: http://review.gluster.org/6468
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>