session2: implement adding streams and responding to reinvites

darcs-hash:20090427155834-b2873-b9b574b543659dbc17d3e752b28c48da5d7336f2
Simplified and improved thread handling in Engine by using threading mod

darcs-hash:20090427164325-58103-e4ecf11358c432e3f205fade96d92db158575e4f