Use interface type instead of implementation type when appropriate.

There are a couple of places in code base where we declare a field / variable as implementation type (i.e. HashMap, HashSet) instead of interface type (i.e. Map, Set), while in other places we do the opposite by declaring as interface type. A quick check indicates that most if not all of these places could be updated so we have a consistent style over the code base (prefer using interface type), which is also a good coding style to stick per best practice.

See more info on https://github.com/apache/zookeeper/pull/102