Change changes file and comment for 7189, for making it 0.2.4-only for now

changes/bug7189_server_only

   o Minor bugfixes:
     - Only disable TLS session ticket support when running as a TLS
-      server.  It's important for clients to remain hard to distinguish
-      from regular firefox connections. Fixes bug 7189; bugfix on
-      Tor 0.2.3.23-rc.
+      server. This keeps clients harder to distinguish from regular firefox
+      connections. Fixes bug 7189; bugfix on Tor 0.2.3.23-rc.

src/common/tortls.c

@@ -1193,8 +1193,12 @@ tor_tls_context_new(crypto_pk_t *identity, unsigned int key_lifetime,
   /* Disable TLS tickets if they're supported.  We never want to use them;
    * using them can make our perfect forward secrecy a little worse, *and*
    * create an opportunity to fingerprint us (since it's unusual to use them
-   * with TLS sessions turned off).  Clients need to advertise support for
-   * them, though to avoid a TLS distinguishability vector.
+   * with TLS sessions turned off).
+   *
+   * In 0.2.4, clients advertise support for them, though to avoid a TLS
+   * distinguishability vector.  This can give us worse PFS, though, if we
+   * get a server that doesn't set SSL_OP_NO_TICKET.  With luck, there will
+   * be few such servers by the time 0.2.4 is more stable.
    */
 #ifdef SSL_OP_NO_TICKET
   if (! is_client) {