Allow pausing IO scheduler from inside core

To handle backups the UIs have to make sure they do stop the IO
scheduler and also don't accidentally restart it while working on it.
Since they have to call start_io from a bunch of locations this can be
a bit difficult to manage.

This introduces a mechanism for the core to pause IO for some time,
which is used by the imex function.  It interacts well with other
calls to dc_start_io() and dc_stop_io() making sure that when resumed
the scheduler will be running or not as the latest calls to them.

This was a little more invasive then hoped due to the scheduler.  The
additional abstraction of the scheduler on the context seems a nice
improvement though.

src/webxdc.rs

@@ -421,7 +421,9 @@ impl Context {
                  DO UPDATE SET last_serial=excluded.last_serial, descr=excluded.descr",
                 paramsv![instance.id, status_update_serial, status_update_serial, descr],
             ).await?;
-            self.interrupt_smtp(InterruptInfo::new(false)).await;
+            self.scheduler
+                .interrupt_smtp(InterruptInfo::new(false))
+                .await;
         }
         Ok(())
     }