Fixup LazyLeafOperations

Fixup LazyLeafOperations

LazyLeafOperations.isEnabled() allows code to query whether leaf nodes
are being treated as expendable, hence assumptions about them being
equal on identity can be violated. We retrofit the check into tests we
modified before.

Furthermore users of getValue() expect it to work with identities,
even when it is being specialized to a Collection. The expectation here
is that the same collection will be returned from NormalizedNode,
which we are violating. Fix this up by introducing a new implementation
class, which hides this difference and defers equality to the backing
map.

JIRA: YANGTOOLS-1019
Change-Id: Icf62619c3ea5a1c994c6547a0674b36d707248d3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>