Data Tree: Enforce 'mandatory true' leaf presence

RFC6020 defines the conditions under which a leaf marked as mandatory needs to be present in https://tools.ietf.org/html/rfc6020#section-7.6.5.

This enforcement should be part of the DataTree.prepare() phase and should validate the resulting tree. Since prepare() cannot be performed concurrently and mandatory leaves are not common, we need to maintain a cache of children paths which must be present under a particular NormalizedNodeContainer. The cache should be global and weakly-keyed on SchemaNode.