cluster/afr: Hybrid Halo mounts

Summary:
- Implements "Hybrid" halo mounts which perform write FOPs synchronously
  to all regions and read FOPs asynchronously to those nodes within the
  region.
- Leverages the fact that the inode cache stores "hints" as to the
  clean/dirty state of upto 16 replicas in the cluster; upon refresh of
  an inode we can do a fresh lookup to get a "wise" node if none exist
  in our region
- Activated via the cluster.halo-hybrid-mode option

Test Plan:
- Run AFR prove tests
- Run halo prove tests

Reviewers: kvigor, sshreyas

Reviewed By: kvigor, sshreyas

FB-commit-id: aca760757afd45e4de2e28dc31b87a73ee52f12d

Change-Id: Ic6728ce93b7b96e3151dccdebccd30e007f4750c
Signed-off-by: Kevin Vigor <kvigor>
Reviewed-on: http://review.gluster.org/16306
CentOS-regression: Gluster Build System <jenkins.org>
NetBSD-regression: NetBSD Build System <jenkins.org>
Smoke: Gluster Build System <jenkins.org>
Reviewed-by: Shreyas Siravara <sshreyas>