A Bridge Authority should compute flag thresholds

As a bridge authority, before we create our networkstatus document, we
should compute the thresholds needed for the various status flags
assigned to each bridge based on the status of all other bridges. We
then add these thresholds to the networkstatus document for easy access.
Fixes for #1117 and #9859.

src/or/dirserv.h

@@ -51,6 +51,7 @@ int list_server_status_v1(smartlist_t *routers, char **router_status_out,
 int dirserv_dump_directory_to_string(char **dir_out,
                                      crypto_pk_t *private_key);
 char *dirserv_get_flag_thresholds_line(void);
+void dirserv_compute_bridge_flag_thresholds(routerlist_t *rl);
 
 int directory_fetches_from_authorities(const or_options_t *options);
 int directory_fetches_dir_info_early(const or_options_t *options);