Shorten wait time until pending pipeline is started

[michaelsauter]

When a pipeline run is triggered for a repository where this is already a running pipeline, the new pipeline run is queued (created in pending state), and a polling mechanism is kicked off. When polling detects no pipeline is running anymore, the pending one is started. The design may waste time up to the polling interval (30s) before starting the pending pipeline. We could improve this wait time by signalling the polling mechanism from the end of a running pipeline that we are done soon, and shorten the polling interval temporarily.