Handle Telegram message deletions that happen before bridging the message is finished

[tulir]

Currently deletions of unknown messages are ignored, but in some cases the messages are unknown because they're in the process of being bridged. In those cases, the messages are bridged successfully, but never get deleted on Matrix

[emanuele-f]

This includes, in particular, messages which are deleted automatically by spam-prevention telegram bots. Currently such messages require manual deletion on the site matrix side