JoinPendingMLSConversationsManager
Joins pending MLS conversations by external commit after this client may have been left out of MLS groups: it ran out of key packages, or it registered its MLS client after conversations were migrated to MLS.
Runs only while incremental sync is live, so every Welcome already sent to this client has been processed and only conversations still in a pending state are joined. The flag is cleared after a successful join, otherwise it is retried the next time sync becomes live.