prop224: Synchronize spec with implementation (August 2017)

During implementation and reviewing we noticed a few more deviations of the implementation from the spec, and also some spec errors.

Here is a list:

1. RENDEZVOUS2 is not identical to RENDEZVOUS1 since rend points strips out the rend cookie.

2. We use a replaycache for the rendcookie when parsing intro2 on the service-side

3. No legacy INTRODUCE cell anymore since that was not needed.

4. We can have multiple intro onion keys with different types on the HS desc (see #22979 (moved))

5. We need to mention the ed25519 base point in the spec since we use it hardcoded in the code.

We might get more items here as client-side review moves forward.