Add Ability to Test and Reconnect to Data Store

[mikeflynn]

I've been using this library as part of a service and I've seen it lose the Redis connection, so I added a simple method on the store (and the related interfaces) to query and reset the connection as needed.

This could apply to any other data store over the network, but could also just be a nil function for stores that don't need a connection reset.

[eaigner]

Thanks for the PR. Wouldn't it be better though to let shield automatically reconnect instead of exposing a TestConnection to the user?

[mikeflynn]

Yup, I had a similar thought...but I ended up not going that route as we would have to have a new setting to tweak how often it should check/reset and would add a thread to check for the reset timer even for applications that were just running in a one off and didn't need it. In the end, I figured I would just offload that logic to the apps that decided they wanted it, but it could certainly go the other way.