Cache default-valued key leaf items

https://tools.ietf.org/html/rfc7950#section-7.8.2 specifies that all leaves which are part of a key must be given a value, disregarding any 'default' statements, making them effectively mandatory.

This is unfortunate for the case where there is a composite key, with one of the leaves having a 'default' value: here the use case is that the default value represents a common value, such as path-id=0 for non-add-path BGP routes, being used when no other information is present.

In a typical deployment, there may be a significant number, if not vast majority, of list items, which have path-id set to this default value, hence it would make sense to use a single LeafNode to represent them.

Teach binding-dom-codec to watch for such keys and maintain a cache, possibly integrated with MDSAL-407, of LeafNodes for use in this scenario.

Note that comparing values can lead to significant overheads, hence the cache should probably use identity checks to discover candidates for deduplication – which leads to the question of how would a Binding application discover the identity objects.