Downgrade ZooKeeper (#506)
Downgrade ZooKeeper (#506)

Curator 4.3.0 depends on ZooKeeper 3.5.7.
So we should align the version.

Modifications:
- Zookeeper 3.6.0 -> 3.5.7
- Armeria 0.99.1 -> 0.99.2
- json-unit 2.16.0 -> 2.16.2
- Spring Boot 2.2.5.RELEASE -> 2.2.6.RELEASE
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