#2194: Memory increase during regridding

Comment (by Roland Haas):

Even though https://bitbucket.org/eschnett/carpet/pull-requests/23/rhaas-commstate-free-buffers/diff does not seem to fix all issues reported here, it did help at least in one case on my workstation running Miguel’s example. As such I would like to commit it after 2019-07-13 unless objected.