Speed up cache key computation.
Speed up cache key computation.

This adds quite a bit of overhead to the general query processing
since there'll often be over a thousand of these per HTTP request (and
several HTTP requests per page draw and several clients waiting for
time in the cache processors).

Before:

  BenchmarkCacheKeying      5000        410395 ns/op

After:

  BenchmarkCacheKeying     10000        151015 ns/op

dustin/seriesly,dustin/seriesly,trigrass2/seriesly,trigrass2/seriesly