Upgrade curator

[jihoonson]

I talked with @gianm offline and raise this PR because he can't afford to work on #4752 right now.
The reason of the test failures is that curator-test 4.0 is not compatible with zookeeper 3.4.x. It looks to be recommended to use 2.12.0 instead (https://issues.apache.org/jira/browse/CURATOR-428).

Also, I excluded zookeeper dependency from curator-recipes to enable soft-compatibility mode (https://curator.apache.org/zk-compatibility.html).

The expected benefits are

• Curator 4.0.0 contains a fix for CURATOR-394 (see also Downgrade Curator. #4103).
• Fixes Another day, another Apache Curator bug #4056, fixes Curator leader election breaks in the overlord when zookeeper has issues #3837.

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This change is 

[drcrallen]

Can you please add the benefits of a 4.0.0?

Taking on a .0 major release is a bit scary

[jihoonson]

The benefits are described in the original PR (#4752). I updated the PR description.

[gianm]

@jihoonson could you try merging master once again; #4787 should help with the CI failures.

[jihoonson]

@gianm thanks. Merged master.

[gianm]

Personally, I am +1 on this, although if any ZK related issues crop up in testing the 0.11 release candidate then we should be ready to roll it back.

I think it's worth the risk to put this out there for the first release candidate, since the update does fix real bugs that people have hit, and the risks at this point are theoretical.

[himanshug]

@gianm @jihoonson sounds good to upgrade. it would be good to run this on a test cluster with some load for a day or so that any trivial issues surface immediately.

[drcrallen]

@himanshug is that something you are equipped to test?

[himanshug]

@drcrallen I would be fine to merge it if @jihoonson has done sanity verification on some cluster with multiple machines. once merged, we can update our metrics cluster to latest master and let it run there .. see if any problems spring up.

[jihoonson]

@himanshug thanks. I'll let you know after testing on our cluster.

[jihoonson]

@himanshug this patch works well on our cluster.

[gianm]

I'll merge this, based on comments so far and testing from @jihoonson. If anyone notices any issues after more testing, that may be related, then please speak up.