Improve performance of is_server_admin by adding a cache

[anoadragon453]

Fixes #18738

Adds a cache to is_server_admin so this bit of code doesn't hammer the DB so hard:

synapse/synapse/events/utils.py

Lines 593 to 600 in 5106818

	# Force-enable server admin metadata because the only time an event with
	# relevant metadata will be when the admin requested it via their admin
	# client config account data. Also, it's "just" some `unsigned` fields, so
	# shouldn't cause much in terms of problems to downstream consumers.
	if config.requester is not None and await self._auth.is_server_admin(
	config.requester
	):
	config = make_config_for_admin(config)

100000 was chosen as large homeservers such as matrix.org easily have that many users syncing within a reasonable time-frame.

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