Fix realtime channel close, reopen channels with active subscriptions

Realtime kept spamming close requests when no subscriptions were left for a topic.

It now only send one close request and waits for the server to acknowledge it.

Additionally, if a channel is closed by the remote, but still has active subscriptions, it will try to reopen it. This helps e.g. when changing users.

Realtime will still refuse to add subscriptions to closing or closed channels, changing that is a bit more involved.