In particular, remove the libasyncns import, which was only used by
GUnixResolver, which is only used when threads are not available.
Likewise remove GWin32Resolver, and the hacky broken non-threaded
parts of GIOScheduler.
https://bugzilla.gnome.org/show_bug.cgi?id=616754
Higher-level wrappers around GResolver. GSocketConnectable provides an
interface for synchronously or asynchronously iterating multiple
socket addresses, with GNetworkAddress and GNetworkService providing
interfaces based on hostname and SRV record resolution.
Part of #548466.
GResolver provides asynchronous (and synchronous-but-cancellable) APIs
for resolving hostnames, reverse-resolving IP addresses back to
hostnames, and resolving SRV records. Part of #548466.
