sip_session2: make add_stream() asynchronous again

darcs-hash:20090423091617-b2873-186f87f3823628538c4eca971625a3f6a19289ca
sip_session2: consistently don't print the tracebacks for a number of exceptions

darcs-hash:20090423092457-b2873-1d967c6ea7c1a6b870cc3b008d09c1f597d5b3f9