[core] reorder member variable declarations

This commit changes the order of member variable declarations in
certain classes to group them by size/alignment requirement and
also to avoid having the first member variable of class to inherit
from `InstanceLocator` for a class which its itself a sub-class of
`InstanceLocator` (this is to avoid extra memory padding added for
empty base class).

This commit also re-defines some of the public member variables in
`Ip6` and `Commissioner` classes as a private and provides getter
methods to access them.

src/core/api/message_api.cpp

@@ -178,8 +178,8 @@ void otMessageGetBufferInfo(otInstance *aInstance, otBufferInfo *aBufferInfo)
     aInstance->mThreadNetif.GetIp6().GetSendQueue().GetInfo(aBufferInfo->mIp6Messages,
                                                             aBufferInfo->mIp6Buffers);
 
-    aInstance->mThreadNetif.GetIp6().mMpl.GetBufferedMessageSet().GetInfo(aBufferInfo->mMplMessages,
-                                                                          aBufferInfo->mMplBuffers);
+    aInstance->mThreadNetif.GetIp6().GetMpl().GetBufferedMessageSet().GetInfo(aBufferInfo->mMplMessages,
+                                                                              aBufferInfo->mMplBuffers);
 
     aInstance->mThreadNetif.GetMle().GetMessageQueue().GetInfo(aBufferInfo->mMleMessages,
                                                                aBufferInfo->mMleBuffers);