fix: always emit AccountsBackgroundFetchDone

A second background_fetch() while one is already running returned
without emitting the event, and the FFI returned 1 for it,
so a UI waiting for an event hangs dc_get_next_event().
Emit the event in any case, so waiting for it is safe.

deltachat-jsonrpc/src/api.rs

@@ -278,7 +278,8 @@ impl CommandApi {
 
     /// Performs a background fetch for all accounts in parallel with a timeout.
     ///
-    /// The `AccountsBackgroundFetchDone` event is emitted at the end even in case of timeout.
+    /// The `AccountsBackgroundFetchDone` event is emitted at the end even in case of timeout,
+    /// and immediately if another background fetch is already running.
     /// Process all events until you get this one and you can safely return to the background
     /// without forgetting to create notifications caused by timing race conditions.
     async fn background_fetch(&self, timeout_in_seconds: f64) -> Result<()> {

deltachat-jsonrpc/src/api/types/events.rs

@@ -394,11 +394,15 @@ pub enum EventType {
         msg_id: u32,
     },
 
-    /// Tells that the Background fetch was completed (or timed out).
-    /// This event acts as a marker, when you reach this event you can be sure
-    /// that all events emitted during the background fetch were processed.
+    /// Tells that a background fetch call is done:
+    /// the fetch completed, timed out, was stopped or was not started.
     ///
-    /// This event is only emitted by the account manager
+    /// For the call that started the fetch, this event acts as a marker:
+    /// all events emitted during the fetch were processed once it is reached.
+    /// A call made while another background fetch is running gets the event immediately,
+    /// and the running fetch keeps emitting events until its own marker.
+    ///
+    /// This event is only emitted by the account manager.
     AccountsBackgroundFetchDone,
     /// Inform that set of chats or the order of the chats in the chatlist has changed.
     ///