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/receive_imf.rs

@@ -363,6 +363,7 @@ pub(crate) async fn receive_imf_inner(
             chat_id.emit_msg_event(context, *msg_id, incoming && fresh);
         }
     }
+    context.new_msgs_notify.notify_one();
 
     mime_parser
         .handle_reports(context, from_id, sent_timestamp, &mime_parser.parts)