cluster/afr: Non-destructive GFID unsplit brain support for v3.6.x

Summary:
- v3.6.3 port of non-destructive GFID unsplit-brain code, almost a re-write for AFR2, but the original behavior lives on.
- This feature allows the GlusterFS filesystem to automagically resolve GFID splitbrain situations by choosing the authorative file based on the last modification time.  Other policies such as majority or size are also possible but not implemented just yet.
- Core feature to Halo Geo-Replication, as this (gfid) form of split-brain is an everyday possibility with async mounts, so there needs to be an automated & scalable method to resolve them via the SHD or optionally in-line by FUSE clients or NFS daemons.
- Operational notes:
  1. Files or directory entries are supported, you can even write files into a directory and they will not be lost.
  2. Streamed writes to a files are fully supported while a split-brain resolution happens,  i.e. the writes will not be interrupted while the unsplit takes place.
  3. Un-split (ones which are determined not to be "authoritative") files are renamed like so: ".<filename>_<random uuid>"

Test Plan:
- Run prove -v tests/basic/gfid_unsplit.t
- Test output: https://phabricator.fb.com/P20041740

Reviewers: moox, dph, sshreyas

Reviewed By: sshreyas

Differential Revision: https://phabricator.fb.com/D2479409

Signature: t1:2479409:1443208319:373218aa9758a1b48db23ea5e211ec303fa92e64

Blame Revision: Change-Id: I5b3d2e79fad74b4372c02b86219e8ee98f5e29dc

Change-Id: I8ef719bcccb19ab6674647e02b72e1b36155fed9
Signed-off-by: Jeff Darcy <jdarcy@fb.com>
Reviewed-on: https://review.gluster.org/17720
Smoke: Gluster Build System <jenkins@build.gluster.org>
Tested-by: Jeff Darcy <jeff@pl.atyp.us>
Reviewed-by: Jeff Darcy <jeff@pl.atyp.us>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>

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