[MINOR] stream_interface: add functions to support running as internal/external tasks

[MINOR] stream_interface: add functions to support running as internal/external tasks

It will soon be necessary to have stream interfaces running as part of
the current task, or as independant tasks. For instance when we want to
implement compression or SSL. It will also be used for applets running
as stream interfaces.

These new functions are used to perform exactly that. Note that it's
still not easy to write a simple echo applet and more functions will
likely be needed.