Wait on multiple MRs if the MRs have different target branches

If two MRs have different target branches (e.g. main and release-1.X), then marge-bot should enqueue both of then (since they don't conflict with each-other).

This should probably be configurable since some setups may not be able to handle the load of multiple full CI runs (e.g. parallelism: 0 for as many as possible and parallelism: 2 for a maximum of two merge queues).

I would be willing to implement this, but I assume it is complicated due to the current way marge polls for results?