lib: istream-multiplex - Forward i_stream_switch_ioloop() to parent

lib: istream-multiplex - Forward i_stream_switch_ioloop() to parent

Most istreams do this because istream_private.parent is set to the parent
stream, but this can't be done with istream-multiplex. The main problem
with attempting to do the same with istream-multiplex is that the different
channels don't share the same I/O. Just because one channel received data
doesn't mean that other channels received any data. (It would be possible
to solve this by implementing a new method that allows overriding
i_stream_set_io(), but I'm not sure if that's a good idea either.)