Improve the memory footprint of the structures which hold a parent's child references

Since working with node paths requires for any given node loading the parent hierarchy which holds the node (to be able to determine the path segments), ModeShape will load from ISPN the children references array for each parent and store that in memory.

Currently the code uses ModeShape's common LinkedListMultimap implementation which is convenient in terms of the API, but which has a larger memory footprint than a regular LinkedHashMap because it holds on to much more data.

This enhancement is about analyzing & changing the code of the child references implementation and trying to see if simpler (smaller) data structures can be used.