IUserTimeZoneService no longer caches user requests

[deanmarcussen]

this pr #16614 has made some changes to how the users time zones are cached which makes the cache useless.

this code in UserTimeZoneSelector now retrieves the current user on every request, before trying to get a timezone for the user.

(or in fact if a timezone calcation is done more than once on a page, which is a common usage, then it queries for the user multiple times)

        var currentUser = await _userManager.GetUserAsync(_httpContextAccessor.HttpContext.User);

the previous arrangement where UserTimeZoneService held the cache for the database query to lookup the user (to get their timezone) was a useful cache. Now it's irrelevant, because the componet calling into that cache is doing all the work.

[Skrypt]

Remembers me something I had fixed once.

[sebastienros]

We need to remove the cache from the IUserTimeZoneService implementation and move it to the UserTimezoneSelector instead, which will prevent it from loading the user. The key will be the username which is fetched from the HttpContext.

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[sebastienros]

@deanmarcussen I suggest you watch today's meeting when it's available, we review the latest PR. My suggestion is to remove this service and put whatever it does (caching) in UserTimeZoneSelector. I explain my reasons during the meeting. The other option we have is to keep what @MikeAlhayek has done in the new PR, which fixes the caching issue (like my suggestion it's a breaking change).

[MikeAlhayek]

@sebastienros additionally, removing the IUserTimeZoneService will be a breaking change and can't be done in 2.x. unless we want to leave it there as useless code for 2.x and remove it in 3.0.