First I began with a set of candidates:

  The client's _required_ list starts with all the protocols
  supported by every release in 0.2.9-stable through current
  master.

  The client's _required_ list starts with all the protocols
  supported by every release in 0.3.5-stable through current
  master.

  Everybody's _recommended_ list starts with all the protocols
  supported by every release in 0.3.5-stable through current master.

Then I removed the protocol versions that we do not actually want to
require or recommend:

  DirCache=1 (deprecated)
  HSDir=1, HSIntro=1-3, HSRend=1 (deprecated)
  (All HS* protocol requirements for clients)
  Link=1-3 (deprecated)
  LinkAuth=1 (obsolete)
  Relay=1 (obsolete)

Make it possible to specify multiple ConsensusParams torrc
lines.

Now directory authority operators can for example put the
main ConsensusParams config in one torrc file and then add to it
from a different torrc file.

Closes ticket 40164.

  Cons=1 is the old format of consensuses, without ed25519 keys. It
  is no longer required or recommended.

  Cons=2 is the new format of consensuses, with ed25519 keys. It
  is now required.

  (Similarly for Desc=1,2 and Microdesc=1,2)

No supported client or relay versions should be affected by this
change, since these versions are supported by clients and relays
running 0.2.9 and later.  It will only take effect once enough
authorities vote for it.

Closes ticket 40162.

This should make diffs easier to read.

In brief: we go through a lot of gymnastics to handle huge protover
numbers, but after years of development we're not even close to 10
for any of our current versions.  We also have a convenient
workaround available in case we ever run out of protocols: if (for
example) we someday need Link=64, we can just add Link2=0 or
something.

This patch is a minimal patch to change tor's behavior; it doesn't
take advantage of the new restrictions.

Implements #40133 and proposal 318.

David Goulet authored Jul 01, 2020 and Nick Mathewson committed Oct 07, 2020

If at least one service is configured as a version 2, a log warning is emitted
once and only once.

Closes #40003

Signed-off-by: David Goulet <dgoulet@torproject.org>