Server/audience validation

[znewton]

Description

If Nexus is rebooted without cleaning up `ClientManager's client list, the next client that joins will receive stale data. This is currently cleaned up by Deli for write clients after 5 min of idle time, but it is never cleaned up for Read clients.

We can leverage Socket.io's internal sockets and rooms maps (synchronized across server instances by Redis adapter) to validate the ClientManager list when a client joins.

Reviewer Guidance

I still need to validate whether socket.io list is more reliable than ClientManager.