HHH-4317 Memory leak in EnumType class

<classname>org.hibernate.type.EnumType</classname> used a static map to cache the <type>enum</type> values, because getting the value of the <type>enum</type> by reflecting is expensive. However, this implementation had the potential to cause memory leaks in come circumstances, especially when using Hibernate in the application server. This is because the cached objects would never be garbage-collected, so the Map acting as cache kept growing infinitely. Instead of using a global <type>enum</type> cache, Hibernate now uses a transient array for each <classname>EnumType</classname> instance to cache the enum values. This improves performance and eliminates the memory leak.