fix: take timestamp_rcvd into account in estimate_deletion_cnt

This did not affect actual message deletion,
because select_expired_messages already takes timestamp_rcvd
into account and does not delete system messages
that say "Messages are end-to-end encrypted" too early.

So it is a minor bug as estimate_deletion_cnt
is meant to only roughly estimate the number of messages
to be deleted. Still, there were no tests before,
so now estimate_deletion_cnt is tested.

src/message.rs

@@ -2100,10 +2100,11 @@ pub async fn estimate_deletion_cnt(
         .count(
             "SELECT COUNT(*)
              FROM msgs m
-             WHERE m.id > ?
-               AND timestamp < ?
-               AND chat_id != ?
-               AND chat_id != ? AND hidden = 0;",
+             WHERE m.id > ?1
+               AND timestamp < ?2      -- Sorting timestamp may be 0 for system messages
+               AND timestamp_rcvd < ?2 -- so we check 'received' timestamp as well.
+               AND chat_id != ?3
+               AND chat_id != ?4 AND hidden = 0;",
             (
                 DC_MSG_ID_LAST_SPECIAL,
                 threshold_timestamp,