soup-address: handle IPv6 literals with scope IDs

Use GNetworkAddress to try to parse IPv6 addresses with scope IDs,
rather than just letting the scope ID get dropped.

Part of https://bugzilla.gnome.org/show_bug.cgi?id=669724
soup-address: fix a GMainContext bug

SoupAddress's GSocketAddressEnumerator implementation was passing the
wrong GMainContext to soup_address_resolve_async(), although it only
ended up affecting the address-is-already-resolved case. Fix this.

Also, remove the async_lookups list from SoupAddressPrivate, and just
make multiple calls to the resolver if there are multiple calls to
soup_address_resolve_async() at once, since that code didn't handle
different GMainContexts correctly.