Rework DataNodeContainerModificationStrategy child tracking

Rework DataNodeContainerModificationStrategy child tracking

Since we do not want to expand the entire modification tree, but
instantiate it lazily, we have used a Guava LoadingCache to load
children.

Unfortunately this has couple of disadvantanges, namely:
- memory overhead of the cache and its entries
- access through a ConcurrentHashMap
- synchronized entry loading

As it turns out, we can do much better by maintaining an immutable
map ourselves, with compare-and-swap locking. This results in
much lower memory overhead as well as fast access, as we perform
only a single volatile read in that case.

JIRA: YANGTOOLS-950
Change-Id: I7b8c6d6561d65373e54cf5a5dd9f0bc2537531ac
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>