The root of the issue comes to the fact that sub-data pointers were
used to match strips before/after copy-on-write. The undo system might
re-use sub-data pointers after re-allocating them, making it so that,
for example, pointer used by sound strip is later re-used by video
strip.
This fix takes an advantage of recently introduced per-sequence UUID
and uses it to match sequences before/after copy-on-write.
We decided to use our own map data structure in general for better
readability and performance.
Reviewers: sergey
Differential Revision: https://developer.blender.org/D7987
