Deprecate worker plugin overwrite policy

[jrbourbeau]

Current when a worker plugin is registered with the same name as an already registered plugin, we do not overwrite the previously registered plugin.

distributed/distributed/worker.py

Lines 2724 to 2725 in 1999c15

	if name in self.plugins:
	return {"status": "repeat"}

However, with nanny plugins this behavior is different and we do overwrite previously registered plugins with the same name. To avoid user confusion, this PR proposes we deprecate our current worker plugin overwrite behavior to match the existing nanny plugin semantic. This will be, I think, more intuitive and will maintainable (xref #5127 (comment)).

[jrbourbeau]

I'll plan to leave this open for a couple more days. If there's no objection, I'll merge this in so it will be in the next release. After that, the plan would be to merge in something like #5142 which actually modifies worker plugin repeat behavior