Bitmex-trio-websocket

* Upgrade slurry-websocket for better closure handling.

The websocket is now forced to connect when the open_bitmex_websocket context is entered. Previously, the websocket
would lazily wait for a subscription to be added, before it would connect. This creates problems because the user will
probably want to know if the connection has succeeded, before any subscriptions are attempted. Especially so, if those
subscriptions require an authenticated connection.

* Upgrade slurry-websocket
* Add check for websocket status, before adding a subscription. There was a check previously, but it would always succeed.
* Slightly more verbose logging on startup and shutdown.

* Upgrade slurry

* Upgrade slurry

* Will now raise BitMEXWebsocketApiError on any error message. This is a breaking change. Up to now, all errors were simply discarded. No error handling is attempted. Any error will cause the connection to be terminated and will raise BitMEXWebsocketApiError.

* Upgrade slurry