Find a way to accelerate creating a keyed hasher

[tkaitchuck]

Generating two random numbers each time a hasher is created is a high overhead.
Rust's default approach is to use the thread local storage to generate a random value and then for each subsequent hasher increment the number.
This is not ideal, because it means if you learn one seed you learn them all, it also requires a hash map lookup each time you want to instantiate a hash map. (Which is not a big deal if that were fast, but right now that involves SipHash which is MUCH slower.)

[tkaitchuck]

This is partially resolved with LocationBasedState.