repeated calls to /login kills performance

This issue has been migrated from [#5647](https://github.com/matrix-org/synapse/issues/5647).

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A common pattern for scripts is to call `/login` for each invocation. The problem is that /login is quite expensive, as we have to notify every server in every room about the new device. This gets worse as you accumulate more and more devices.

`/login` is rate-limited, so there's a limit to the damage you can cause, but still, it's possible to impair the server performance, and accumulate thousands of devices.

Perhaps we need a more granular rate-limit (only 10 new devices per day, or something?) Or some other way of discouraging people from calling /login frequently?

Related: #4934, #2704