geo-rep: Avoid cold tier bricks during ENTRY operation

This is a series of patch which aims to fix geo-replication
in a Tiering Volume.

Problem:
Consider, a file is placed in volume initially and then hot tier is
attached. During any operation on the file, due to lookup a linkto
file is created in hot tier.

Now, any namespace operation carried out on the file is recorded in
both cold and hot tier.
There is a room for races when both changelogs are replayed.

Solution:
So, We are going to replay (namespace related)operations
only in the hot tier.

Why?
a. If the file is directly placed in Hot tier , all fops will be
recorded in HOT tier.
b. If  the file is already present in Cold tier, and if any fop is
carried out, it creates linkto file in Hot tier.
Now, operations like UNLINK, RENAME are captured in Hot
tier(by means of linkto file).

This way, we can get both tier's operation in HOT tier itself.

Now, once the file is demoted to COLD tier, any namespace operation
carried out on the cold tier can be avoided as we directly RECORD
the same in HOT tier.

How?
1. Check whether the brick is cold tier and skip ENTRY operation.
2. Also, if it is cold tier brick, use Xsync(which is used during initial run).
   This will help in getting all cold tier bricks changes using File System crawl
   and helps in avoiding races with hot tier brick(which can happen
   if historychangelog used in cold tier brick).

Dependent patches:
1. http://review.gluster.org/12239
2. http://review.gluster.org/12326

Change-Id: I7692b1dbb8813a7e253451bca02f8f09a5782dde
BUG: 1266875
Signed-off-by: Saravanakumar Arumugam <sarumuga@redhat.com>
Reviewed-on: http://review.gluster.org/12355
Tested-by: NetBSD Build System <jenkins@build.gluster.org>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Aravinda VK <avishwan@redhat.com>

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