AthenaKernel+CxxUtils: Change overlap handling of ConcurrentRangeMap.

We were allowing ranges in ConcurrentRangeMap to overlap one another. This isn't just academic, as we can frequently get such ranges from IOVDbSvc. However, this was leading in some cases to a lookup failure.

Change the semantics of overlap/extend so that we no longer overlap ranges. Rather, in case of a overlap, the new range is truncated to avoid the overlap. (We also allow treating an overlapping final range as an extension.)

See ATLASRECTS-5028.