Remove predictability and hashing

[robert-milan]

Removes the use of hashing to predict where an object should be added or where to start searching the slabs for that object. We don't need this in Metrictank because the string interning library uses an inverted index to store the addresses of all added objects.

Note: TestAdd1Mil was not included in this PR because it was only used for comparison.

(pprof) top 20
Showing nodes accounting for -62.86s, 68.68% of 91.53s total
Dropped 200 nodes (cum <= 0.46s)
      flat  flat%   sum%        cum   cum%
   -32.20s 35.18% 35.18%    -63.22s 69.07%  github.com/replay/go-generic-object-store.(*slabPool).add
   -16.41s 17.93% 53.11%    -16.41s 17.93%  github.com/replay/go-generic-object-store.(*slabPool).getNextID (inline)
   -15.07s 16.46% 69.57%    -15.07s 16.46%  github.com/replay/go-generic-object-store/vendor/github.com/willf/bitset.(*BitSet).Test (inline)
     0.83s  0.91% 68.67%      0.83s  0.91%  github.com/replay/go-generic-object-store/vendor/github.com/willf/bitset.(*BitSet).NextClear
    -0.02s 0.022% 68.69%    -16.43s 17.95%  github.com/replay/go-generic-object-store.(*slabPool).getNextSlabID (inline)
     0.01s 0.011% 68.68%    -63.11s 68.95%  github.com/replay/go-generic-object-store.TestAdd1Mil
         0     0% 68.68%    -63.23s 69.08%  github.com/replay/go-generic-object-store.(*ObjectStore).Add
         0     0% 68.68%    -63.11s 68.95%  testing.tRunner