Previously the registered callbacks on pipeline actions were only executed when these actions were triggered from web-console. For example, you couldn't follow the change stream if a pipeline was started through the API.
Now the occurred pipeline actions are derived from each pipeline's status transitions, received when polling /v0/pipelines.
Aside from the functional changes, the new implementation has lower coupling to adjacent code than the previous one.

Fix #4822: skip the PAUSED state if we don't need to listen for any changes