Revert "Use async name resolving to improve net thread responsiveness"

This reverts commit caf6150e9785da408f1e603ae70eae25b5202d98.

getaddrinfo_a has a nasty tendency to segfault internally in its
background thread, on every version of glibc I tested, especially
under helgrind.

See https://sourceware.org/bugzilla/show_bug.cgi?id=20874