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>

CHANGELOG.md

@@ -17,7 +17,7 @@
   "setup changed" message into all chats we share with the peer #3187
 - do not delete duplicate messages on IMAP immediately to accidentally deleting
   the last copy #3138
-- speed up loading of chat messages #3171
+- speed up loading of chat messages #3171 #3194
 - clear more columns when message expires due to `delete_device_after` setting #3181
 - do not try to use stale SMTP connections #3180
 - Slightly improve finding the correct server after logging in #3207