Use recently introduced BKE_fcurve_merge_duplicate_keys (that was moved
from the transform system to BKE) to merge keyframes on the same frame
after snapping (same as what would happen with the transform system).
This makes behavior consistent and prevents a state after snapping that
cannot be reproduced in any other way.
NOTE: same probably has to be done for greasepencil, but that is for
another commit.
b5ebc9bb24