add central certificate storage

[na4ma4]

This PR will eventually lead to ACME support, but because multiple replicas of honeycomb can run in a swarm I think there needs to be an ability to centrally store the certificates either ACME or static.

This would also mean that the proxy wouldn't need restarts to add or remove certificates.

This PR is going to grow before it gets out of draft status.

[na4ma4]

So far redis has been added, I had some preliminary setup for etcd but redis/memcached could be lighter and easier to deploy next to honeycomb.

There is a cache as well, so if the redis is not available the service can continue to run until it returns.

[na4ma4]

I changed PrimaryProvider into a slice so that multiple providers can be used, not sure if that's how you'd want it to work.

[jmalloc]

I changed PrimaryProvider into a slice so that multiple providers can be used, not sure if that's how you'd want it to work.

I'd probably implement a new provider that searches through a slice of other providers in order, rather than changing the logic here. Just from a naming perspective it doesn't make sense to have more than one "primary" provider.