Reconstruct Merge Train immediately when master is advanced

Problem

Currently, when master is advanced, the merge train will be reconstructed when the latest pipeline in the first merge request of the merge train finished.

However, since there are no points to wait the pipeline result from merge train PoV, we should reconstruct it immediately.

Proposal

Create a new hook event to reconstruct merge train when target branch is advanced.

Considerably, we should cancel all of the current pipelines before creating new pipelines.