Stop printing solver stats after solving finishes

[sim642]

Another thing that I noticed a while ago, but what's relevant to the case from the issue description (where the deadlock is during warning output): We never turn off the solver stats signal, so it keeps printing even after the solver has finished and we're doing some other (long) post-processing, e.g. g2html, witnesses, or I guess just printing warnings. An obvious thing (but not complete fix) would be to actually remove the interval signaling after solving is done.

Originally posted by @sim642 in #1781