Track schema tree generator linkage

Runtime information requires looking up target generator for a schema
tree particant, which means that AbstractCompositeGenerator needs to
resolve QName to the original instantiation.

We already have that mechanics for the most part, as we need to perform
the lookup to resolve augment statements target.

Introduce SchemaTree{Child,Parent} decomposition of the addressing
problem, and retrofit generators to support them. This will allow us to
support lookups along the schema tree axis.

JIRA: MDSAL-696
Change-Id: I619fe9e7c0896d6d3f0f33b04d5200ca8ec30340
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>