fts: Don't reindex FTS mails if .cache file is deleted

This means that if fts is enabled, "doveadm index" no longer adds mails
to dovecot.index.cache if it's deleted. However, it was rarely used for
that purpose. More likely due to a corrupted cache file all the mails were
unnecessarily being opened and reindexed.

src/plugins/fts/fts-storage.c

@@ -105,9 +105,10 @@ fts_mailbox_get_status(struct mailbox *box, enum mailbox_status_items items,
 		if (fts_mailbox_get_last_cached_seq(box, &seq) < 0)
 			return -1;
 
-		/* use whichever is smaller */
-		if (status_r->last_cached_seq > seq)
-			status_r->last_cached_seq = seq;
+		/* Always use the FTS's last_cached_seq. This is because we
+		   don't want to reindex all mails to FTS if .cache file is
+		   deleted. */
+		status_r->last_cached_seq = seq;
 	}
 	return 0;
 }