Replace libeio with a generic threadpool for getaddrinfo()

libeio isn't designed as a generic thread pool, and is not really
suitable for getaddrinfo().

References:
  http://lists.schmorp.de/pipermail/libev/2009q2/000663.html
  http://lists.schmorp.de/pipermail/libev/2013q1/002067.html
  http://lists.schmorp.de/pipermail/libev/2013q1/002073.html
Copyright year bump