Separate out AbstractNodeContainerModificationStrategy abstract methods

AbstractNodeContainerModificationStrategy currently requires subclassing to provide two methods:

• createEmptyValue()
• createBuilder()

All these methods perform the same basic thing: they check argument type and invoke a a builder method, with some special-casing to handle top-level map nodes.

It seems we could easily provide a base class, which would dispatch to based on a method handle (or similar). This would reduce the need to subclass the strategy, simply by providing a common object to implement these methods. Two distinct implementations with 9 total objects, increasing the size of a particular apply strategy. For this we'll end up with a bimorphic invocation at the call site (which is merge operations, so not overly critical), which can be easily be inlined, offering a probably equally-performant-or-faster bridge.