An advanced hash table supporting configurable garbage collection semantics
of keys and values, optional referential-equality, full concurrency of
retrievals, and adjustable expected concurrency for updates.
This is an internal data structure designed for very specific needs;
it will most often be used as a replacement for EnumMap, although
the focus on the Enum aspect is modelled as an int primitive:
think of using the ordinals of an Enum to simulate the EnumMap.
For efficient lookup based on Class types as key,
a ClassValue should be used; however it requires
lazy association of values; this helper wraps
a plain HashMap but optimises lookups via the ClassValue.