Don't update value if it is already set to the same value

[MorrisJobke]

• this PR makes sure to warm up the cache for that user
• then the logic within the "if is in cache" code can be used to reduce needed queries
• inspired by @andreas-p's PR: Skip updating unchanged user settings #2128
• it also fixes a not thrown exception if the precondition is not met (wasn't covered by the unit tests because the caches aren't warmed and thus this code wasn't executed then)

cc @rullzer @LukasReschke @nickvergessen

[mention-bot]

@MorrisJobke, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rullzer, @icewind1991 and @bartv2 to be potential reviewers.

[rullzer]

As discussed 👍

[andreas-p]

IMHO this should either:

• use the return value of getUserValue(...) instead of accessing the userCache, or
• call the dedicated prewarm method $this->getUserValues($user), which will perform getDIInit() itself.

[rullzer]

@andreas-p fair enough.

It now uses the value from the getUserValue function.
The FixDI is still there because this is a nasty hack anyway which we need to get rid of at some point.

[icewind1991]

👍 looks good