libpqxx 7.10.7: backporting a bit more internal wiring

Still eliminating some internal things that might cause problems like in #1195 (a crash on shutdown on some systems in some builds, hard to reproduce). It's possible that 7.10.6 already fixed all these problems, but just in case, we'll take a bit more of the related changes I made in 8.0 and backport them to 7.10.