The private data is allocated at the same time as RedsStream and
goes immediatly after the main RedsStream data.
This private member will allow to hide internal RedsStream
implementation details from the rest of spice-server.
SASL authentication mostly use members from RedsStream to do its work, so
it makes sense to have its code in reds_stream.c. This should allow to make
RedsStream::sasl private in the future.
The AsyncRead structure in reds.h wraps an async read + callback to
be done on a stream. Moving it to reds_stream.h is needed in order
to move SASL authentication there.
Now that stream creation and SSL enabling are done by helpers
in reds_stream.c, we can move the initialization of the vfunc
read/write pointers there too.
Initializing a new stream means initializing quite a few fields.
This commit factors this initialization in a dedicated reds_stream_new
helper. This also helps moving more code from reds.c to reds_stream.c
