Fix helix test issues.

[zzmao]

Increate timeout threshold for helix balance verifier.
Add onBecomeDroppedFromOffline in HelixVcrStateModel.

[codecov-io]

Codecov Report

Merging #1177 into master will decrease coverage by 0.07%.
The diff coverage is 20%.

@@             Coverage Diff              @@
##             master    #1177      +/-   ##
============================================
- Coverage     69.86%   69.79%   -0.08%     
+ Complexity     5374     5370       -4     
============================================
  Files           428      428              
  Lines         32863    32865       +2     
  Branches       4147     4147              
============================================
- Hits          22960    22938      -22     
- Misses         8772     8789      +17     
- Partials       1131     1138       +7

Impacted Files	Coverage Δ	Complexity Δ
...ava/com.github.ambry.cloud/HelixVcrStateModel.java	61.53% <0%> (-8.03%)	5 <0> (ø)
...rc/main/java/com.github.ambry.cloud/VcrServer.java	76.34% <50%> (+0.81%)	4 <0> (ø)	⬇️
...c/main/java/com.github.ambry.network/Selector.java	77.22% <0%> (-3.48%)	47% <0%> (-1%)
...github.ambry.rest/AsyncRequestResponseHandler.java	88.59% <0%> (-2.29%)	23% <0%> (ø)
...in/java/com.github.ambry.store/BlobStoreStats.java	71.13% <0%> (-0.62%)	103% <0%> (ø)
...va/com.github.ambry.replication/ReplicaThread.java	74.15% <0%> (-0.57%)	64% <0%> (-2%)
...java/com.github.ambry.network/SSLTransmission.java	69.42% <0%> (-0.32%)	68% <0%> (-1%)
...in/java/com.github.ambry.network/SocketServer.java	84.01% <0%> (+0.4%)	15% <0%> (ø)	⬇️

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