For automatic topic subscriptions, I plan to introduce a third
subscription state to indicate them. This patch includes minor tweaks
I wanted to add while working on that:
* Introduce constants instead of numbers
* Remove a TODO that doesn't seem like a good idea any more
* Remove a `"length": 1` on sub_state that did not do anything
(but it might have been meant to indicate that it was supposed
to be a boolean, which would be wrong)
Bug: T284836
Change-Id: I6e6096968ad38510102287bccd349090b6ca4280
Per Manuel Arostegui in T263817#7033384. The limit is 5000.
(I picked it arbitrarily, there's no real rationale for it.)
Also log a warning when any user reaches half of the limit,
so that we might make a decision about changing this mechanism
before it starts affecting users. Maybe at that time we'll
have data to show that it's safe to remove the limit.
Bug: T263817
Change-Id: I18a8ee0ad7383759229c5721d5253fb591457d4d
