Introduce new ZNode type: container

BACKGROUND
============
A recurring problem for ZooKeeper users is garbage collection of parent nodes. Many recipes (e.g. locks, leaders, etc.) call for the creation of a parent node under which participants create sequential nodes. When the participant is done, it deletes its node. In practice, the ZooKeeper tree begins to fill up with orphaned parent nodes that are no longer needed. The ZooKeeper APIs don’t provide a way to clean these. Over time, ZooKeeper can become unstable due to the number of these nodes.

CURRENT SOLUTIONS
===================
Apache Curator has a workaround solution for this by providing the Reaper class which runs in the background looking for orphaned parent nodes and deleting them. This isn’t ideal and it would be better if ZooKeeper supported this directly.

PROPOSAL
=========
ZOOKEEPER-723 and ZOOKEEPER-834 have been proposed to allow EPHEMERAL nodes to contain child nodes. This is not optimum as EPHEMERALs are tied to a session and the general use case of parent nodes is for PERSISTENT nodes. This proposal adds a new node type, CONTAINER. A CONTAINER node is the same as a PERSISTENT node with the additional property that when its last child is deleted, it is deleted (and CONTAINER nodes recursively up the tree are deleted if empty).

CANONICAL USAGE
================

while ( true) { // or some reasonable limit
    try {
        zk.create(path, ...);
        break;
    } catch ( KeeperException.NoNodeException e ) {
        try {
            zk.createContainer(containerPath, ...);
        } catch ( KeeperException.NodeExistsException ignore) {
       }
    }
}