imap: call delete_expired_imap_messages() less often

This operation takes roughly 0.3 s on a moderate size database.
Calling it once before scanning all folders and scanning
the watched folder instead of each time after downloading
a message from a folder speeds up IMAP loop.

CHANGELOG.md

@@ -12,6 +12,7 @@
 
 ### Fixes
 - fix splitting off text from webxdc messages #3032
+- call slow `delete_expired_imap_messages()` less often #3037
 
 
 ## 1.72.0