bakery: remove Request type.

httpbakery: remove Service type. Add cookie helpers.

Also change bakery.DischargeAll so that it returns a slice suitable
for putting in a cookie.
It is already documented in the godoc doc: "All the discharge macaroons 
will be bound to the primary macaroon."

Yes, binding twice would break things. But I don't *think* that should
be an issue - you can't usefully add caveats to a macaroon after
discharging it, so I think that doing the binding here makes sense.
If it turns out it's actually useful to be able to discharge but not bind,
we can always create a DischargeAllUnbound function in the future.

I'm hoping that no-one will need to call Bind directly any more.