pipeline: don't re-add active tasks and copy immediately

[lyakh]

In a topology with two connected pipelines, e.g. two playback pipelines, connected on a mixer, if one PCM is already streaming and then the other one is started too, two bugs trigger:

1. when adding pipelines to the scheduler, all pipelines are added, including any common pipelines behind the mixer
2. the mixer pipeline then gets to run first, while it has no data available to it yet. To fix these bugs
3. don't re-add any active pipelines
4. when a pipeline is added for streaming, let it begin copying immediately instead of skipping a period.