Vespa ZooKeeper server interface

Subdirectory zookeeper-server has code for the zookeeper server
version we use by default, testing out new versions can be done by
copying zookeeper-server dir to zookeeper-server-<version>,
updating CMakeLists.txt and pom.xml in that directory and pom.xml and
CMakeLists.txt in this directory. Selecting zookeeper server version
that is used runtime is done with feature flag ZOOKEEPER_SERVER_VERSION.