SI-6961 no structural sharing in list serialization

Revert list serialization back to what it was in 2.9.x and before.

Partial revert of a6fcd70b60 e234978dfd, which fixed SI-5374.
The ListBuffer part of the fix remains in place.
Merge pull request #2093 from adriaanm/ticket-6961

SI-6961 no structural sharing in list serialization