Better handling of DoS attacks on onion services

We have received various reports on attackers being able to DoS onion services in various ways. Examples:

a) Layer-7 attacks where the attacker spams HTTP requests: https://www.hackerfactor.com/blog/index.php?/archives/777-Stopping-Tor-Attacks.html b) DoS through the Tor protocol (intense circuit construction #16052m legacy/trac#15515 (moved)).

We should come up with designs and plans on how to mitigate those DoS attacks better in the future.

Due to the anonymous unlinkable nature of Tor onion service clients, these designs should be modular enough so that onion service operators can write their own anti-DoS modules to handle specific cases of attacks.

This is a parent ticket to handle the various subtasks.