Implement naive flow control in IN channel for RPC/Proxy

This is required to make attachments sending work.

It sends a FlowControlAckWithDestination RTS RPC command
if the local available window is lower than the half of the window size.

This makes the sender to know it has room to send more PDUs.
Be more pep8 complaint

See https://www.python.org/dev/peps/pep-0008/ for details.
RTS cookies are transmitted using little endian

So we have to set it up the UUID objects properly
to use it in RTS RPC commands
Only RPC PDUs are subject to flow control

If all incoming data were treated as read, then the client
closes the connection as the data was inconsistent

   bytes_received > bytes_sent