Move list/key checks to onStatementAdded()

Move list/key checks to onStatementAdded()

We do not want to use the rabbit hole to StmtContext during effective
build, but rather perform the check as soon as the statement is added.

Depending on how the model is structured action/notification declaration
might happen before the key's presence is established and therefore
we perform an eager check first, but pay attention to ancestor's state.

If we encounter an ancestor which has not completed FULL_DECLARATION,
we hook an inference check to run just before it does.

JIRA: YANGTOOLS-1186
Change-Id: I8d8871a0eae860ba2327d05c43355a7ee3ffd382
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>