Change servers during processing of queries

Hello.

I want to fix the bug in io.js: https://github.com/iojs/io.js/issues/894
Same in node.js: https://github.com/joyent/node/issues/9243

Problems happen when function ares_change_servers() called before all
requests will be completed, or in other words, during the execution of the
function processfds().

In this patch new dns requests are handled by the new servers, and the old
servers closed when all requests will be resolved.

Thank you.