Zookeeper 3.5.6 and rollback for Windows client (3.4.14)

[DavidVujic]

Description

This PR is aiming to solve issues for Windows clients.

Mac OS X and Linux will use ZooKeeper 3.5.6 - the current latest stable version. Windows clients will use version 3.4.14, because of compatibility issues between client and server as described in issue #222. I think this could be an issue in the C client and will investigate it in a separate issue.

To be able to support two Minor versions of ZooKeeper, the logger in the C++ Wrapper node-zk.cpp had to be rewritten. The logger API is different between the versions.

Also, Node.js 8 has been removed from the Travis CI build. I think supporting the two latest LTS versions of Node.js is good enough.

Got feedback?

Motivation and Context

Fixes issue #222

How Has This Been Tested?

Travis CI build OK
Docker integration tests, examples, GCC and unit tests passed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).