Zookeeper master detector for v1

[keis]

the zookeeper integration only seems to be for v0. What is the recommended way for detecting the mesos master with v1?

[jdef]

A while back there was discussion of refactoring the ZK-based leadership detector to expose a simpler interface, and that as part of the refactoring effort the newer code would land in a separate repo, independent of the mesos-go project. That never happened and so people still rely on the variant in the v0/ lib.

I still like the idea of separating the leader detector from v1 implementation. The proposal outlined in #339 is nice that sense - the exposed interface is pretty simple and allows for arbitrarily complex (possibly 3rd party) implementations under the hood.