Fix deadlock issue with custom Enum implementation

[stevedlawrence]

Our Enum implementation use for property values/lookups has important
benefits over Scala Enumerations. However, it seems like the way we
implement it can lead to circular deadlocks. When a Value is initialized
it causes the Enum to be initialized. And the Enum initialized calls
forceConstruction to initialized all the Values. This leads to circular
instantiating that works in most cases, but sometimes leads to a
deadlock. Where this deadlock exists or how to fix it is not clear. It's
also not clear what changed to make this deadlock much more likely.

To avoid this, we this change removes the forceConstruction function and
replaces it with a manually managed Array of Enum Values. This Array is
lazily evaluated so that instantiated an Enum does not also directly
instantiate the Enum Value, avoiding the circular deadlock. This does
require extra code to define an Enum, but the majority of Enums are in
generated code so doesn't add much maintenance burden.

DAFFODIL-2704

[tuxji]

+1

Thanks for fixing this issue.

[mbeckerle]

Looks good.
+1