  o Major bugfixes (security):
    - Resist a hashtable-based CPU denial-of-service attack against
      relays. Previously we used a naive unkeyed hash function to look up
      circuits in a circuitmux object. An attacker could exploit this to
      construct circuits with chosen circuit IDs in order to try to create
      collisions and make the hash table inefficient.  Now we use a SipHash
      construction for this hash table instead. Fixes bug 40391; bugfix on
      0.2.4.4-alpha. This issue is also tracked as TROVE-2021-005.
      Reported by Jann Horn from Google's Project Zero.
