core: improve Conflicts.checkSpacingRequirements in stdcm context

[eckter]

Related to scaling stdcm with large timetables, see also https://github.com/osrd-project/osrd-confidential/issues/519

When checking if the added has conflicts in the current block, we do as follow:

1. We iterate over the newly generated zone requirements for the new train
2. For each zone requirement, list of all scheduled train requirements for the given zone
3. We add the new requirement to the list
4. We find any conflict between any pair of requirement in the list
5. We filter out the ones that don't affect the new train

Steps 3-4 are done this way to reuse conflict detection code, but they scale very poorly with large timetables. We end up spending half of the global computation time in this method.

The first obvious fix would be to pre-filter the requirements (see #7649) and it would help a lot, but I don't think it would be quite enough. We may have to write a different version of this method that doesn't check for every pair of conflict.