HTTP layer: add API to pre-allocate a socket / pre-SSL handshake

API to pre-DNS lookup is already there. If it is known that an app will contact 1 server and later another one, latency could be reduced by starting an SSL handshake to the 2nd server before the resource is requested.

E.g. the 1st request from any Facebook app will go to graph.facebook.com, but consecutive requests will be loaded from a static image server. We could connect the socket to the image server right after sending the request to graph.facebook.com, and have it setup when the request to it is going out (because the 1st request will probably return some JSON containing links to other servers).

Also pre-allocating more sockets to the same server (for Facebook: graph.facebook.com) should give performance benefits.