Research approaches for improving the availability of services under DoS

We've been improving the health of the network during onion service DoS, but not the onion service availability. This is a task for looking at this angle.

During the related Stockholm session we looked into various approaches that could help us towards that goal. Here are some of them:

• Introducing application-layer anonymous tokens that allow legit clients to get priority over DoS attacker
• PoW approaches like argon2
• CAPTCHA approaches like introducing a token server giving reCAPTCHA tokens
• Hiding introduction points by rate limiting how quickly clients can find them. Valet nodes?
• Having intros check that clients don't use the same IP over and over. Proof-of-existence?
• Pay bitcoin to introduce

Each of the above solutions has problems and this is a ticket to investigate at least the most promising of them, and attempt to move forward with something.