Getters/setters for padding fields on ACF messages

Looking at the current code base I noticed in #182 the getters/setters for `subtype` and `acf_msg_length` have been removed. Shouldn't we for consistency and reduce code size also remove the explicit getters/setters for the padding field? I think these are pretty useless if we don't have getters/setters for `acf_msg_length` as well.

Since we introduced the `_SetPayloadLength(...)` function which take care of setting both `acf_msg_length` and `pad` for ACF messsages, I think it would be consistent to also remove `_GetPad(...)` and `_SetPad(...)`

People would still be able to modify these values directly using the `GetField(...)` and `SetField(...)` functions if they really want to.