quick-read: refactor

- peel out 'open behind' functionality into a separate translator
- issue where, if file size had grown by revalidate, data was not flushed
- removed unnecessary acquistion of table->lock (e.g in qr_lookup())
- keep inode ctx persistent, prune only data (effectively changing the
  order of lock acquisition from INODE -> TABLE)
- validation with readdirplus
- use variable size iobufs to simply cached reads

Change-Id: If1586d0298fd1697ddff9fd7008efb3d286d436a
BUG: 846240
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4403
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>