Ignore liveness update of current node if it is actually alive

[jsjtzyy]

When starting ambry-server, HelixClusterManager is instantiated before
participating into cluster. During this short window, Helix may send
notification that makes this server mark itself is down. This brings
Disk_Unavailable errors when handling replication requests and checking
the state of disk(disk state depends on node state). Such error is
misleading because disk is actually good and the server is able to serve
replication request. Hence, this PR makes node ignore liveness update of
itself if it is actually alive. (It should be safe, because other
frontends and servers will mark it down and no subsequent requests will
be routed to it)

[codecov-io]

Codecov Report

Merging #1264 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master   #1264      +/-   ##
===========================================
+ Coverage     72.35%   72.4%   +0.05%     
- Complexity     6160    6163       +3     
===========================================
  Files           444     444              
  Lines         35364   35364              
  Branches       4491    4491              
===========================================
+ Hits          25586   25606      +20     
+ Misses         8602    8587      -15     
+ Partials       1176    1171       -5

Impacted Files	Coverage Δ	Complexity Δ
...m.github.ambry.clustermap/HelixClusterManager.java	86.5% <100%> (+0.25%)	43 <1> (+1)	⬆️
.../main/java/com.github.ambry.store/ScanResults.java	81.25% <0%> (-1.57%)	16% <0%> (-1%)
...va/com.github.ambry.replication/ReplicaThread.java	86.38% <0%> (-0.95%)	90% <0%> (-1%)
...java/com.github.ambry.router/GetBlobOperation.java	90.73% <0%> (-0.72%)	39% <0%> (ø)
...java/com.github.ambry.store/CompactionManager.java	91.66% <0%> (-0.6%)	25% <0%> (ø)
...ain/java/com.github.ambry.router/PutOperation.java	91.55% <0%> (-0.17%)	113% <0%> (-1%)
...in/java/com.github.ambry.network/SocketServer.java	84.29% <0%> (+0.41%)	15% <0%> (ø)	⬇️
...rc/main/java/com.github.ambry.store/BlobStore.java	90.46% <0%> (+0.46%)	98% <0%> (+2%)	⬆️
...in/java/com.github.ambry.store/BlobStoreStats.java	70.83% <0%> (+1.45%)	103% <0%> (+1%)	⬆️
... and 4 more

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[lightningrob]

LGTM. I would shorten the explanation about the disk errors.

[lightningrob]

I don't think we need this much explanation of what we saw without the fix. It's enough to say that the list of live instances doesn't include this node since it hasn't joined yet.