cluster/afr: fsync() guarantees POST-OP completion

AFR now provides a stronger guarantee that fsync() returns only
after completely finishing all the deferred/delayed POST-OP on that
open file.

To acheive this we make a stub out of the returning fsync and
register it with the "delayed" frame in afr_changelog_wake_resume().

The delayed frame, after getting woken up and finishing the POST-OP
will call_resume() the registered stub (which UNWINDs the fsync) at
the time of frame destruction.

This provides a guarantee that an application's (or FUSE) fsync()
returns only after finishing up all the previous transactions,
including delayed POST-OPs and UNLOCK.

Change-Id: Iaa955457e2f25088a144fde37ad0444277b5cf49
BUG: 927146
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4737
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>