Remove the timestamp from AUTHENTICATE cells; replace with random bytes

This isn't actually much of an issue, since only relays send
AUTHENTICATE cells, but while we're removing timestamps, we might as
well do this too.

Part of proposal 222.  I didn't take the approach in the proposal of
using a time-based HMAC, since that was a bad-prng-mitigation hack
from SSL3, and in real life, if you don't have a good RNG, you're
hopeless as a Tor server.

changes/no_client_timestamps_024

@@ -3,8 +3,10 @@
       not used for anything, and they provided one small way for clients
       to be distinguished from each other as they moved from network to
       network or behind NAT. Implements part of proposal 222.
-    - Clients now round timestamps in INTRODUCE2 cells to the nearest
+    - Clients now round timestamps in INTRODUCE2 cells down to the nearest
       10 minutes.  If a new Support022HiddenServices option is set to 0,
       or if it's set to "auto" and the feature is disabled in the consensus,
-      the timestamp is sent as 0 instead.
-
+      the timestamp is sent as 0 instead. Implements part of proposal 222.
+    - Stop sending timestamps in AUTHENTICATE cells. This is not such
+      a big deal from a security point of view, but it achieves no actual
+      good purpose, and isn't needed. Implements part of proposal 222.

src/or/connection_or.c

@@ -2287,19 +2287,11 @@ connection_or_compute_authenticate_cell_body(or_connection_t *conn,
   if (server)
     return V3_AUTH_FIXED_PART_LEN; // ptr-out
 
-  /* Time: 8 octets. */
-  {
-    uint64_t now = time(NULL);
-    if ((time_t)now < 0)
-      return -1;
-    set_uint32(ptr, htonl((uint32_t)(now>>32)));
-    set_uint32(ptr+4, htonl((uint32_t)now));
-    ptr += 8;
-  }
-
-  /* Nonce: 16 octets. */
-  crypto_rand((char*)ptr, 16);
-  ptr += 16;
+  /* 8 octets were reserved for the current time, but we're trying to get out
+   * of the habit of sending time around willynilly.  Fortunately, nothing
+   * checks it.  That's followed by 16 bytes of nonce. */
+  crypto_rand((char*)ptr, 24);
+  ptr += 24;
 
   tor_assert(ptr - out == V3_AUTH_BODY_LEN);