closes https://github.com/TryGhost/Arch/issues/85
- Added a cache configuration option to signal "reuse of redis connection" for Redis cache adapter. The connection reuse it turned on by default to be shared between caches. They rely on unique "keyPrefix" structure, so there is no collision side-effects when reusing same Redis Store.
- The Redis connection options like "ttl" are shared with the first connection that's crated. So if there's a need to have unique configuration, a separate connection has to be created by passing `"reuseConnection": false` parameter
