Race condition when synchronizing user lists

[dpp-gerrit]

Host: A
Client: B
Client: C
Client: D

D invites E

First bug: The host does not know anything as long as D has not synchronized the user lists, but E does !

Second bug:

Instead of D, A invites E

While synchronizing the user lists, following can happen:

A tells E that B,C and D are part of the session. E now may send an activity to B,C and D (e.g its color).

B,C and D does not know anything about E yet, discarding the activity and blowing up their ActivitySequencer for E, because when
E sends another activity (now B,C and D may know E), the sequencer is still waiting for the former activity which was discarded. The sequencer stalls
for E unless a timeout occured AND E sends another activity.

Reported by: kargor

Original Ticket: dpp/bugs/758