Replace MemObject clients list with std::list

[yadij]

Removing use of custom dlink_list type.

[yadij]

FYI: this has not yet faced any traffic for run-time testing and past additions of CbcPointer have had subtle unexpected free's which will need to be checked for. I would like to defer expansion of CbcPointer use for store_client to later PRs, but if necessary that may happen here.