Added concern for a faster "cache_key" method

This concern provides an optimized/simplified version of the "cache_key"
method. This method is about 9 times faster than the default "cache_key"
method.

The produced cache keys _are_ different from the previous ones but this
is worth the performance improvement. To showcase this I set up a
benchmark (using benchmark-ips) that compares FasterCacheKeys#cache_key
with the regular cache_key. The output of this benchmark was:

    Calculating -------------------------------------
               cache_key     4.825k i/100ms
          cache_key_fast    21.723k i/100ms
    -------------------------------------------------
               cache_key     59.422k (± 7.2%) i/s -    299.150k
          cache_key_fast    543.243k (± 9.2%) i/s -      2.694M

    Comparison:
          cache_key_fast:   543243.4 i/s
               cache_key:    59422.0 i/s - 9.14x slower

To see the impact on real code I applied these changes and benchmarked
Issue#referenced_merge_requests. For an issue referencing 10 merge
requests these changes shaved off between 40 and 60 milliseconds.