Fix disconnects breaking view updates for other connections

[joshua-spacetime]

Description of Changes

Fixes a bug in client disconnect logic that would mark a client's views as dropped(unsubscribed). However it was marking the identity's views as dropped, not the connection. So if an identity had multiple connections open, each subscribing to different views, and one of them disconnected, the subscriptions for the other connections would break. The observed behavior would be that they would stop receiving subscription updates. This could potentially lead to their client cache getting into a corrupted state.

Now, instead of dropping all of the views for a particular identity on disconnect, we drop only the views for that particular connection. And when I say drop, what I really mean is decrement. A view is not dropped completely unless it no longer had any subscribers.

API and ABI breaking changes

None

Expected complexity level and risk

2

Testing

Regression smoketest was added

[gmorenz]

That sounds like a good find, but I don't think it explains the issue I'm seeing in #4603. I have a different identity for each connection because I'm starting them with .with_token(None::<String>). And I just confirmed that

println!("{}", player1.conn.identity());
println!("{}", _player2.conn.identity());

prints two different things.