fix: Add tolerance to MemberListTimestamp (#5366)

Let's add a 1-minute tolerance to `Params::MemberListTimestamp`.

This adds to the group membership consistency algo the following properties:
- If remote group membership changes were made by two members in parallel, both of them are applied,
  no matter in which order the messages are received.
- If we remove a member locally, only explicit remote member additions/removals made in parallel are
  allowed, but not the synchronisation of the member list from "To". Before, if somebody managed to
  reply earlier than receiving our removal of a member, we added it back which doesn't look good.

src/constants.rs

@@ -219,6 +219,10 @@ pub(crate) const DEFAULT_MAX_SMTP_RCPT_TO: usize = 50;
 /// How far the last quota check needs to be in the past to be checked by the background function (in seconds).
 pub(crate) const DC_BACKGROUND_FETCH_QUOTA_CHECK_RATELIMIT: u64 = 12 * 60 * 60; // 12 hours
 
+/// How far in the future the sender timestamp of a message is allowed to be, in seconds. Also used
+/// in the group membership consistency algo to reject outdated membership changes.
+pub(crate) const TIMESTAMP_SENT_TOLERANCE: i64 = 60;
+
 #[cfg(test)]
 mod tests {
     use num_traits::FromPrimitive;