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Alembic import: fixed memory management for CoW copies of CacheFile blocks

When a CacheFile datablock is copied to an in-main datablock, it means that
the new copy should be independent of the old copy. In this case certain
properties are reset/duplicated. When it is copied to a no-main datablock,
we assume this is a copy-on-write copy that's used for evaluating things,
in which case the handle and object paths are referenced instead of
reset/ duplicated. This prevents us from re-opening the Alembic file every
time a new CoW copy is made.

The same approach is taken when freeing data (only when an in-main datablock
is freed do we actually free the data).