FieldItemlListInterface::processDefaultValue($default_value) is expected to massage polymorphic data

Split from #2318605: uuid, created, changed, language default value implementations need to be updated.

Problem/Motivation

The current API allows specifying "allowed values" using two different formats:
- a literal, in which case it will be assigned to the first property of the first item.
- a numerically indexed array of items, each item being a property/value array.- NULL or array() for no default value

I.e you can do either one of:

$base_field_definition->setDefaultValue(42);
$base_field_definition->setDefaultValue(array(
  array('value' => 42),
  array('value' => 43),
));

That $default_value gets passed as is (litteral or array) to FIL::processDefaultValue(), which is supposed to "massage" it (e.g transform 'NOW' into the current timestamp).

Problem: all current implementations (including the ones added in #2318605: uuid, created, changed, language default value implementations need to be updated) fail to akcknoledge that polymorphism: they all assume one of the two possible formats, and break if they receive the other.

Proposed resolution

We can't expect each FIL::processDefaultValue() to do the legwork of supporting that polymorphism.
We should normalize the $default_value present in the Definition to the array format before calling FIL::processDefaultValue() on it, so that implementations can do their massaging on a predictable format.

This could be done either:

1) in the base implementation of FieldItemList::processDefaultValues(),
Pros:
- easy...
Cons:
- only works if child implementations think of calling the parent first
- keeps polymorphic input on FIL::processDefaultValue(), which is painful to document (see the curent phpdoc)

2) by the caller of ::processDefaultValues(), before the call.
Pros:
- removes polymorphism on FIL::processDefaultValue() completely, clearer and easier to document
Cons:
- the code that calls ::processDefaultValues() is duplicated in BaseFieldDefinition::getDefaultValue() and FieldConfigBase::getDefaultValue()...

Both approaches remove polymorphism on FDI::getDefaultValue(), since it always returns an array. That's a good thing, polymorphism is nice DX on setters but terrible DX on getters :-)