Before this commit the V2Transport::m_send_buffer is used to store the
garbage:
* During MAYBE_V1 state, it's there despite not being sent.
* During AWAITING_KEY state, while it is being sent.
* At the end of the AWAITING_KEY state it cannot be wiped as it's still
needed to compute the garbage authentication packet.
Change this by introducing a separate m_send_garbage field, taking over
the first and last role listed above. This means the garbage is only in
the send buffer when it's actually being sent, removing a few special
cases related to this.