Do ephemeral deletion in async task background loop (#3194)

* Do ephemeral deletion in background loop

1. in start_io start ephemeral async task, in stop_io cancel ephemeral async task

2. start ephemeral async task which loops like this:

- wait until next time a message deletion is needed or an interrupt occurs (see 3.)
- perform delete_expired_messages including sending MSGS_CHANGED events

3. on new messages (incoming or outgoing) with ephemeral timer:

- interrupt ephemeral async task

* Changelog

* Fix and improve test

* no return value needed

* address @link2xt review comments

* slight normalization: have only one place where we wait for interrupt_receiver

* simplify sql statement -- and don't exit the ephemeral_task if there is an sql problem but rather wait

* Remove now-unused `ephemeral_task` JoinHandle

The JoinHandle is now inside the Scheduler.

* fix clippy

* Revert accidental move of the line

* Add log

Co-authored-by: holger krekel <holger@merlinux.eu>
Co-authored-by: link2xt <link2xt@testrun.org>

src/sql.rs

@@ -599,10 +599,6 @@ impl Sql {
 }
 
 pub async fn housekeeping(context: &Context) -> Result<()> {
-    if let Err(err) = crate::ephemeral::delete_expired_messages(context).await {
-        warn!(context, "Failed to delete expired messages: {}", err);
-    }
-
     if let Err(err) = remove_unused_files(context).await {
         warn!(
             context,