Split _wait and _unref

This also removes BackupProvider::join in favour of implementing
Future directly.  I wondered about implementing a FusedFutre to make
this a little safer but it would introduce a dependency on the futures
crate in deltachat-ffi which did not exist yet, so I didn't do that.

deltachat-ffi/deltachat.h

@@ -2706,13 +2706,21 @@ char * dc_backup_provider_qr_svg (dc_context_t* context, const dc_backup_provide
 /**
  * Waits for the sending to finish and frees the backup provider object.
  *
- * @memberof dc_backup_sender_t
+ * This should only be called once.
+ *
+ * @memberof dc_backup_provider_t
  * @param context The context.
  * @param backup_provider The backup provider object as created by
  *    dc_backup_provider_new().  If NULL is given nothing is done.
  */
 void dc_backup_provider_wait (dc_context_t* context, dc_backup_provider_t* backup_provider);
 
+/**
+ * Frees a dc_backup_provider_t object.
+ *
+ * @memberof dc_backup_provider_t
+ */
+void dc_backup_prvider_unref (dc_backup_provider_t* backup_provider);
 
 /**
  * Gets a backup offered by a dc_backup_provider_t object on another device.

deltachat-ffi/src/lib.rs

@@ -4185,12 +4185,17 @@ pub unsafe extern "C" fn dc_backup_provider_wait(
         return;
     }
     let ctx = &*context;
-    let provider = Box::from_raw(provider);
-    block_on(provider.join())
+    let provider = &mut *provider;
+    block_on(provider)
         .log_err(ctx, "Failed to join provider")
         .ok();
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_backup_provider_unref(provider: *mut dc_backup_provider_t) {
+    drop(Box::from_raw(provider));
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_receive_backup(
     context: *mut dc_context_t,