No longer advertise or negotiate any consensus method before 13.

Implements proposal 215; closes ticket 10163.

Why?  From proposal 215:

   Consensus method 1 is no longer viable for the Tor network.  It
   doesn't result in a microdescriptor consensus, and omits other
   fields that clients need in order to work well.  Consensus methods
   under 12 have security issues, since they let a single authority
   set a consensus parameter.
...
   For example, while Tor 0.2.4.x is under development, authorities
   should really not be running anything before Tor 0.2.3.x.  Tor
   0.2.3.x has supported consensus method 13 since 0.2.3.21-rc, so
   it's okay for 0.2.4.x to require 13 as the minimum method.  We even
   might go back to method 12, since the worst outcome of not using 13
   would be some warnings in client logs.  Consensus method 12 was a
   security improvement, so we don't want to roll back before that.

src/or/dirvote.c

@@ -110,7 +110,8 @@ format_networkstatus_vote(crypto_pk_t *private_signing_key,
     char *params;
     authority_cert_t *cert = v3_ns->cert;
     char *methods =
-      make_consensus_method_list(1, MAX_SUPPORTED_CONSENSUS_METHOD, " ");
+      make_consensus_method_list(MIN_SUPPORTED_CONSENSUS_METHOD,
+                                 MAX_SUPPORTED_CONSENSUS_METHOD, " ");
     format_iso_time(published, v3_ns->published);
     format_iso_time(va, v3_ns->valid_after);
     format_iso_time(fu, v3_ns->fresh_until);
@@ -537,7 +538,8 @@ compute_consensus_method(smartlist_t *votes)
 static int
 consensus_method_is_supported(int method)
 {
-  return (method >= 1) && (method <= MAX_SUPPORTED_CONSENSUS_METHOD);
+  return (method >= MIN_SUPPORTED_CONSENSUS_METHOD) &&
+    (method <= MAX_SUPPORTED_CONSENSUS_METHOD);
 }
 
 /** Return a newly allocated string holding the numbers between low and high
@@ -1346,7 +1348,7 @@ networkstatus_compute_consensus(smartlist_t *votes,
     log_warn(LD_DIR, "The other authorities will use consensus method %d, "
              "which I don't support.  Maybe I should upgrade!",
              consensus_method);
-    consensus_method = 1;
+    consensus_method = MAX_SUPPORTED_CONSENSUS_METHOD;
   }
 
   /* Compute medians of time-related things, and figure out how many

src/or/dirvote.h

@@ -21,6 +21,9 @@
 /** Smallest allowable voting interval. */
 #define MIN_VOTE_INTERVAL 300
 
+/** The lowest consensus method that we currently support. */
+#define MIN_SUPPORTED_CONSENSUS_METHOD 13
+
 /** The highest consensus method that we currently support. */
 #define MAX_SUPPORTED_CONSENSUS_METHOD 18