ssb: Don't schedule duplicate history stream requests for the same account. Changes how we schedule idle work. Let's see if this is better.

src/ssb.h

@@ -799,17 +799,19 @@ JSValue tf_ssb_connection_requests_to_object(tf_ssb_connection_t* connection);
 /**
 ** A function scheduled to be run later.
 ** @param connection The owning connection.
+** @param skip Whether the work ought to be skipped, because it is being replaced.
 ** @param user_data User data registered with the callback.
 */
-typedef void(tf_ssb_scheduled_callback_t)(tf_ssb_connection_t* connection, void* user_data);
+typedef void(tf_ssb_scheduled_callback_t)(tf_ssb_connection_t* connection, bool skip, void* user_data);
 
 /**
 ** Schedule work to be run when the connection is next idle.
 ** @param connection The owning connection.
+** @param key A key identifying the work.  If work by the same key already exists, it will be replaced.
 ** @param callback The callback to call.
 ** @param user_data User data to pass to the callback.
 */
-void tf_ssb_connection_schedule_idle(tf_ssb_connection_t* connection, tf_ssb_scheduled_callback_t* callback, void* user_data);
+void tf_ssb_connection_schedule_idle(tf_ssb_connection_t* connection, const char* key, tf_ssb_scheduled_callback_t* callback, void* user_data);
 
 /**
 ** Schedule work to run on a worker thread.