Hello,I'd like to drop support for Zookeeper 3.4 in 1.15, upgrading the default to 3.5 with an opt-in for 3.6.
Supporting Zookeeper 3.4 (which is already EOL) prevents us from upgrading Curator to 5.x, which would allow us to properly fix an issue with inconsistent state. It is also required to eventually support ZK 3.6.
