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/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,