This commit changes the model for OpenThread classes so that all
classes track/locate their owning/parent `otInstance` object (i.e.,
the owning `otInstance` object reference is expected to be passed as
an input argument in the object constructor). This replaces and
simplifies current model where some of classes track/locate the parent
`ThreadNetif`, `MeshForwarder`, or `Ip6` object references instead.
This helps harmonize the model across OpenThread source and also
simplify the `Locator` class implementation.
This commit updates the `otNetworkDataIterator` to also store the
sub-tlv offset (in addition to main prefix tlv offset and the entry
index). It also updates two methods `GetNextExternalRoute()` and
`GetNextOnMeshPrefix()`. This commit also adds network data unit
test module.
Abtin Keshavarzian