net: do not use send buffer to store/cache garbage

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.

src/test/fuzz/p2p_transport_serialization.cpp

@@ -366,7 +366,7 @@ std::unique_ptr<Transport> MakeV2Transport(NodeId nodeid, bool initiator, RNG& r
              .Write(garb.data(), garb.size())
              .Finalize(UCharCast(ent.data()));
 
-    return std::make_unique<V2Transport>(nodeid, initiator, SER_NETWORK, INIT_PROTO_VERSION, key, ent, garb);
+    return std::make_unique<V2Transport>(nodeid, initiator, SER_NETWORK, INIT_PROTO_VERSION, key, ent, std::move(garb));
 }
 
 } // namespace