We now distinguish more clearly between joining and re-joining a MUC.

The `enterRoom` method has been refactored away.

Instead, `join` or `rejoin` should be used, depending on the circumstances.

Don't fetch cached occupants in the `initialize` function, instead, only fetch
them if we're restoring a MUC that we're still connected to.

If we're not restoring a still-connected MUC, then we clear the
occupants cache, and the messages cache if `clear_messages_on_reconnection` is `true`.

updates #1807
updates #1808