[MINOR] add a ->private member to the stream_interface

[MINOR] add a ->private member to the stream_interface

iohandlers will need to store some form of context and for this will
need a way to find their call context. We add the ->private as well
as ->st0 and ->st1 for that purpose. Most likely ->private will be
initialized to the current session and ->st0 and ->st1 will be used
to maintain any form of internal state between calls.