Enable server fallback for GTalk account

Enable server fallback for GTalk account

Instead of hardcoding one server, let the connection manager try doing
SRV resolution and if the resolution fails, let it try a list of fallback
servers. Note that talkx.l.google.com is now used instead of
talk.google.com. This is the DNS used in the Windows client, this DNS
returns a list of IPs instead of 1. This also enable usage of HTTPS
proxy when falling back to port 443 with old_ssl.

Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>