The request-response behaviour that is used for sending the transfer
proof actually has a functionality for buffering a message if we
are currently not connected. However, the request-response behaviour
also emits a dial attempt and **drops** all buffered messages if this
dial attempt fails. For us, the dial attempt will very likely always
fail because Bob is very likely behind NAT and we have to wait for
him to reconnect to us.
To mitigate this, we build our own buffer within the EventLoop and
send transfer proofs as soon as we are connected again.
Resolves#348.
