The nested socket implementation seems to have never been properly
updated since guac_socket was changed to rely on implementation-specific
buffering. This meant that absolutely every write resulted in a nest
instruction being sent to the parent socket.
Data should instead be built up within the internal buffer, with each
flush writing as much of the internal buffer as possible within a nest
instruction, leaving any partial UTF-8 characters at the end of the
buffer for later completion with future writes.
