network sockets: preallocate sockets if known that more than 1 is needed

Starting the TCP handshake as early as possible will reduce latency. The problem is that we cannot just blindly allocate several sockets to a host, because it might turn out that only one is needed (e.g. HTTP redirect to mobile version of a web site).
Apparently Google does some smart heuristics about whether more than 1 socket is needed, like checking the cache for resources from that host.