ot::Spinel::Encoder::BeginFrame() method

Begins a new spinel property update command frame to be added/written to the frame buffer. If there is a previous frame being written (for which `EndFrame()` has not yet been called), calling `BeginFrame()` will discard and clear the previous unfinished frame. The spinel transaction ID (TID) in the given spinel header is used to determine the priority level of the new frame. Non-zero TID value indicates that the frame is a response and therefore it uses higher priority level. Saves the write position before the property key (see also `SavePosition()`) so that if fetching the property fails and the property key should be switched to `LAST_STATUS` with an error status, the saved position can be used to update the property key in the frame (see also `OverwriteWithLastStatusError()`)