[core] pass/locate otInstance to all OpenThread classes

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.

src/core/api/instance_api.cpp

@@ -87,7 +87,7 @@ otInstance::otInstance(void) :
     mTimerMicroScheduler(*this),
 #endif
     mIp6(*this),
-    mThreadNetif(mIp6),
+    mThreadNetif(*this),
 #if OPENTHREAD_ENABLE_RAW_LINK_API
     mLinkRaw(*this),
 #endif // OPENTHREAD_ENABLE_RAW_LINK_API