object-storage: remove stat of directories

It was painfully slow to gather a list of all the files in a directory tree
when there were as many directories as files, since we also stat'd all of the
directories as well. We only did that to cache using memcache the result, to
know when it changed. However, there was no way for a memcache object to be
passed down to this code. So we have removed the memcache support entirely,
and removed the stat of the directories as a result.

See BZ 911448 (https://bugzilla.redhat.com/show_bug.cgi?id=911448).

Change-Id: I2625f82eca36c31eeffa84dc76ef7f3b48aafec5
Signed-off-by: Mohammed Junaid <junaid@redhat.com>
Reviewed-on: http://review.gluster.org/5252
Reviewed-by: Peter Portante <pportant@redhat.com>
Tested-by: Peter Portante <pportant@redhat.com>