Remove tornado loop kwarg in Server

[fjetter]

Our Server classes and Server subclasses are currently accepting a tornado IOLoop as an argument. When removing the tornado IOLoop from the code base, we'll need to introduce appropriate replacements

• Server likely doesn't need a kwarg. It is always async, we can just use asyncio.get_running_loop
• Client, Cluster, Scheduler, etc. might want to use a LoopRunner instance or a token instead

[fjetter]

@graingert can you please provide a brief summary where we are with this? Is this ticket still relevant? IIUC, we just removed or deprecated this usage and we can likely close this ticket, can't we?

[graingert]

yes this ticket is mostly completed by

#6473
#6443

when working on #6163 I discovered that passing a non-current loop to Scheduler, Server, Worker or Nanny was already broken and so could be deprecated without any disruption.

howerver, the Client and Cluster (and any other SyncMethodMixin) classes, still require either a running loop or None so that they can determine if they are being run from a synchronous or asynchronous 'perspective'

[fjetter]

Great, I'll close this then as a duplicate