SSL, which will be added in an upcoming commit, requires some background
processing, which is best done in a "run" function in our architecture.
This commit adds stream_run() and stream_run_wait() and calls to them from
the places where they will be required.
These invariants are checked by vconn_open() and stream_open(), but there
is no reason not to check them earlier also. vconn and stream creation
don't have to go through vconn_open() and stream_open(), so this ensures
that the invariants get checked either way.
This code is heavily based on the vconn code. Eventually we should make
the stream-based vconns (currently that's all of them) a wrapper around
streams, but I haven't done that yet.
SSL is not implemented yet.
