In LOG, don't allocate strings and make string concatenations if log level is not recorded

[aanno]

I did some profiling on snapcast (while working on https://github.com/badaix/snapcast/issues/1423). From my measurements, I concluded that LOG currently allocates strings and makes string concatenations even if the log level at hand is not recorded.

Hence I tweaked aixlog.hpp to stop this. I also added an caching layer for the decision if a certain message is actually recorded, depending on the log level.

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