Members added by legacy DC set timestamps of all recipients to the timestamp of receiving a message

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When a member using new DC that has new group consistency algorithm #6404 and stores member timestamps gets added to groups by old DC client (or outdated invite bot), they set the timestamps for all members to the date when the message was received. This may result in re-adding members that have left the group before this new DC client received a "Member added" message.

Same may happen if DC client using the latest version with group consistency algorithm gets added to a group by non-DC MUA, but in that case it does not matter as MUA is allowed to add members anyway.

We should set all timestamps to 0 instead because actual timestamps are not known.