New APIs for message processing loops

This patch adds new C APIs
dc_get_next_msgs() and dc_wait_next_msgs(),
and their JSON-RPC counterparts
get_next_msgs() and wait_next_msgs().

New configuration "last_msg_id"
tracks the last message ID processed by the bot.
get_next_msgs() returns message IDs above
the "last_msg_id".
wait_next_msgs() waits for new message notification
and calls get_next_msgs().
wait_next_msgs() can be used to build
a separate message processing loop
independent of the event loop.

Async Python API get_fresh_messages_in_arrival_order()
is deprecated in favor of get_next_messages().

Introduced Python APIs:
- Account.wait_next_incoming_message()
- Message.is_from_self()
- Message.is_from_device()

Introduced Rust APIs:
- Context.set_config_u32()
- Context.get_config_u32()

src/scheduler.rs

@@ -62,6 +62,11 @@ impl SchedulerState {
     /// Starts the scheduler if it is not yet started.
     async fn do_start(mut inner: RwLockWriteGuard<'_, InnerSchedulerState>, context: Context) {
         info!(context, "starting IO");
+
+        // Notify message processing loop
+        // to allow processing old messages after restart.
+        context.new_msgs_notify.notify_one();
+
         let ctx = context.clone();
         match Scheduler::start(context).await {
             Ok(scheduler) => *inner = InnerSchedulerState::Started(scheduler),
@@ -95,6 +100,11 @@ impl SchedulerState {
         // to terminate on receiving the next event and then call stop_io()
         // which will emit the below event(s)
         info!(context, "stopping IO");
+
+        // Wake up message processing loop even if there are no messages
+        // to allow for clean shutdown.
+        context.new_msgs_notify.notify_one();
+
         if let Some(debug_logging) = context.debug_logging.read().await.as_ref() {
             debug_logging.loop_handle.abort();
         }