core: stdcm: improve conflict detection performances

[eckter]

In stdcm on long requests, we spend roughly 70% of the computation time identifying conflicts. And it's not even running the signaling simulation and similar complex things, it's really just listing and comparing requirements. Specifically, IncrementalConflictDetectorImpl.checkConflicts and IncrementalConflictDetectorImpl.analyzeConflicts take respectively 16% + 42% of the global execution time.

We could certainly do better.

Maybe instead of having a MutableList<SpacingZoneRequirement> for each zone, we could use something like a RangeSet, with time ranges where it's free / used.