Initial support for keys following zipfian

[lfsu-ali]

This patch allows memtier generate keys that follows zipfian distribution. An additional parameter --key-zipf-exp is introduced, meaning P(Key = n) ~ n^{-exp}, which is bounded to (0, 5) to be sane. The range of keys are limited to positive in this version.

Signed-off-by: Su Lifan [email protected]