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()

deltachat-ffi/src/lib.rs

@@ -1280,6 +1280,50 @@ pub unsafe extern "C" fn dc_get_fresh_msgs(
     })
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_get_next_msgs(context: *mut dc_context_t) -> *mut dc_array::dc_array_t {
+    if context.is_null() {
+        eprintln!("ignoring careless call to dc_get_next_msgs()");
+        return ptr::null_mut();
+    }
+    let ctx = &*context;
+
+    let msg_ids = block_on(ctx.get_next_msgs())
+        .context("failed to get next messages")
+        .log_err(ctx)
+        .unwrap_or_default();
+    let arr = dc_array_t::from(
+        msg_ids
+            .iter()
+            .map(|msg_id| msg_id.to_u32())
+            .collect::<Vec<u32>>(),
+    );
+    Box::into_raw(Box::new(arr))
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn dc_wait_next_msgs(
+    context: *mut dc_context_t,
+) -> *mut dc_array::dc_array_t {
+    if context.is_null() {
+        eprintln!("ignoring careless call to dc_wait_next_msgs()");
+        return ptr::null_mut();
+    }
+    let ctx = &*context;
+
+    let msg_ids = block_on(ctx.wait_next_msgs())
+        .context("failed to wait for next messages")
+        .log_err(ctx)
+        .unwrap_or_default();
+    let arr = dc_array_t::from(
+        msg_ids
+            .iter()
+            .map(|msg_id| msg_id.to_u32())
+            .collect::<Vec<u32>>(),
+    );
+    Box::into_raw(Box::new(arr))
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_marknoticed_chat(context: *mut dc_context_t, chat_id: u32) {
     if context.is_null() {