CxxUtils+AthenaKernel+StoreGate: Allow CondCont::insert to extend the last range.

Add an option to ConcurrentRangeMap::emplace to allow extending the range of the last entry in the case of a duplicate. We used to do this from WriteCondHandle, but that was racy; do it here so we can do both steps without releasing the lock.

Allow insert operations on CondCont to extend the range of the last entry in the container in the case of a duplicate. We used to do this from WriteCondHandle, but that was racy; do it like this so we can do both steps without releasing the lock.

WriteCondHandle had logic to allow extending the range to the last entry of the container in the case of a duplicate. Doing it here, however, was racy, so this logic has been moved inside CondCont itself.

See !23367 (merged).