timeout connectivity check to 3/4 seconds max

[diamante0018]

The connectivity check can be a bottleneck for several seconds. In my experience, even more than 3 seconds up to 8 seconds.

I think that's bad.

If it takes more than 3 seconds, it should time out, proceed with the main CDN, and start downloading fast. Users' experience is degraded when they have to wait for such a connectivity check for longer than 3 seconds.

[mxve]

Still looking to backport the new cdn rating soon(tm)
It initially rates with a low timeout and only goes higher if no server responded
soon(tm)(tm)