Tracking API speed improvement: cache parsed yaml in tracker cache (DevicesDetection)

[mattab]

Parsing YML files from DevicesDetection plugin takes up to 100ms per Tracking API request. This was rather unacceptable performance... We want to move DevicesDetection to first class citizen in #3962

Stefan implemented a clever fix, to cache the parsed YML arrays in the tracker cache.

This was done in:

a3c01d6

662178b

[mattab]

Now it takes 1 ms or less to load in memory the cached files. good caching FTW

[mattab]

In 96cb532: Refs #4683 Refactor this little bit