Support a sync() request to Zookeeper

[granthenke]

The documentation in the default Zookeeper client is here.

[jzillmann]

So will that be obsolete with #45 then ?

[granthenke]

@jzillmann These are 2 separate "features". #45 is to add a versioned call to delete, this is to support zookeepers sync request. As far as the linked Kafka issue goes, the versioned delete request is what is required. The Kafka issue just got linked here as part of the discussion.