Deprecate worker callbacks

[jrbourbeau]

We have an older Client.register_worker_callbacks system for allowing users to specify a setup function to be run when workers are started. However this system was replaced with the more generic worker plugin system in #2453. Today, Client.register_worker_callbacks is actually implemented using our worker plugin system:

distributed/distributed/client.py

Lines 4565 to 4583 in d0a93e9

	def register_worker_callbacks(self, setup=None):
	"""
	Registers a setup callback function for all current and future workers.
	This registers a new setup function for workers in this cluster. The
	function will run immediately on all currently connected workers. It
	will also be run upon connection by any workers that are added in the
	future. Multiple setup functions can be registered - these will be
	called in the order they were added.
	If the function takes an input argument named ``dask_worker`` then
	that variable will be populated with the worker itself.
	Parameters
	----------
	setup : callable(dask_worker: Worker) -> None
	Function to register and run on all workers
	"""
	return self.register_worker_plugin(_WorkerSetupPlugin(setup))

Deprecating worker callbacks was part of the original plan when the worker plugin system was introduced, I think we just never got around to it. Opening an issue to track deprecating Client.register_worker_callbacks in favor of Client.register_worker_plugin