stream: add pw_stream_drive()

This method is meant for driver streams when they have data or need
data.

Previously only source (output) streams could drive the graph and
indirectly by doing pw_stream_queue_buffer(). This behaviour is now
deprecated (but will still work) in favour of calling
pw_stream_drive() explicitly.

pw_stream_drive() will call the process function immediately (if
driver) or later (when not a driver) so that the same code paths
can be used when running as a driver or not.

In a similar way, a consumer (input) driver stream can now be
written. It will call pw_stream_drive() when it wants data and the
process function will be called when the data is ready.

See #1484