Add HelixVcrCluster and HelixVcrClusterFactory.

[zzmao]

Add HelixVcrCluster and HelixVcrClusterFactory.

[zzmao]

There will be another PR for ReplicationEngine to dynamically add/remove partition.
For now, it's just a one time assignment.

[lightningrob]

First round of review comments. I need to understand the Helix messaging a bit better.

[codecov-io]

Codecov Report

Merging #1158 into master will decrease coverage by 0.05%.
The diff coverage is 72.44%.

@@             Coverage Diff              @@
##             master    #1158      +/-   ##
============================================
- Coverage     69.84%   69.78%   -0.06%     
- Complexity     5316     5332      +16     
============================================
  Files           420      425       +5     
  Lines         32547    32644      +97     
  Branches       4139     4147       +8     
============================================
+ Hits          22732    22782      +50     
- Misses         8695     8724      +29     
- Partials       1120     1138      +18

Impacted Files	Coverage Δ	Complexity Δ
...om.github.ambry.cloud/StaticVcrClusterFactory.java	87.5% <ø> (+9.72%)	2 <0> (ø)	⬇️
.../java/com.github.ambry.cloud/StaticVcrCluster.java	76.19% <0%> (-3.81%)	6 <0> (ø)
...com.github.ambry.cloud/HelixVcrClusterMetrics.java	100% <100%> (ø)	1 <1> (?)
....github.ambry.cloud/HelixVcrStateModelFactory.java	100% <100%> (ø)	2 <2> (?)
...com.github.ambry.cloud/HelixVcrClusterFactory.java	63.63% <63.63%> (ø)	2 <2> (?)
...ava/com.github.ambry.cloud/HelixVcrStateModel.java	69.56% <69.56%> (ø)	5 <5> (?)
...n/java/com.github.ambry.cloud/HelixVcrCluster.java	71.69% <71.69%> (ø)	10 <10> (?)
.../main/java/com.github.ambry.store/ScanResults.java	81.25% <0%> (-3.13%)	16% <0%> (-2%)
...java/com.github.ambry.store/CompactionManager.java	87.33% <0%> (-2.67%)	19% <0%> (ø)
...github.ambry.rest/AsyncRequestResponseHandler.java	88.59% <0%> (-2.29%)	23% <0%> (ø)
... and 11 more

---

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[cgtz]

can just make this ConcurrentHashMap.newKeySet(); it's a static method

[cgtz]

private static final

[lightningrob]

A few minor style comments, we can merge after addressing those.