api: add APIs to stop background fetch

New APIs are JSON-RPC method stop_background_fetch(),
Rust method Accounts.stop_background_fetch()
and C method dc_accounts_stop_background_fetch().

These APIs allow to cancel background fetch early
even before the initially set timeout,
for example on Android when the system calls
`Service.onTimeout()` for a `dataSync` foreground service.

deltachat-jsonrpc/src/api.rs

@@ -283,6 +283,11 @@ impl CommandApi {
         Ok(())
     }
 
+    async fn stop_background_fetch(&self) -> Result<()> {
+        self.accounts.read().await.stop_background_fetch();
+        Ok(())
+    }
+
     // ---------------------------------------------
     // Methods that work on individual accounts
     // ---------------------------------------------