Allow optimistic data on connections to hidden services

Now that legacy/trac#13153 (moved) is close to being resolved, it occurs to me that all supported (0.2.3.1-alpha and later) hidden services know what optimistic data is.

And by a stroke of luck (or who knows, maybe it was planned), it looks like the server-side of the hidden service uses the same code as the exit relays do.

So with a few lines to let clients be willing to do it, we could cut out the initial round-trip to hidden service websites for each request from Tor Browser!