Fixes from https://github.com/markfisher/xdzk/pull/8
Introduced Container domain classes

Added some simple Container domain classes and supporting
interfaces. The ContainerAware interface provides access to
an iterator over all Containers in the cluster. ChildPathIterator
is a generic iterator over children nodes in a Curator/ZooKeeper
path.
Bug fixes

When a SUSPENDED event is raised we cancel the leader selector
and the containers childre cache. If a LOST event is the next
event we attempt another close which results in IllegalStateException.
This exception can be ignored since these objects were no longer
in a usable state.

Fixed an NPE in ContainerListener.

Logged leader interruption at level INFO.