Add simulation testing via turmoil

[LucioFranco]

This adds basic simulation testing via turmoil to mini-redis. This approach uses trait objects to avoid feature flags.

[LucioFranco]

I prefer this approach over the cfg flag version here #112

[carllerche]

IMO, I'm w/ @Darksonn here. I think Turmoil is out of scope for mini-redis. Mini-redis is intended to be one of the first introductions to Tokio. The Tokio tutorial is based on it. Simulation testing, pros and cons, when to use it, etc... are all more advanced topics which should be addressed separately IMO.