Add benchmark for comparing all store implementations

[philippgille]

Some people already have a strong preference for a specific store (for example because they're already running Redis for their web service, so they don't want to set up and manage anything else), but others are open for any new key-value store as long as its performance is great.

We should add a benchmark that compares the different gokv.Store implementations with each other. This will especially help in deciding between read vs write optimized stores, embedded vs remote stores and self-hosted vs cloud-hosted stores.

[recoilme]

This may help: https://github.com/recoilme/pogreb-bench

[philippgille]

@recoilme: Thanks for the advice! I'll have a more detailed look when I get to this issue.