add ability to define a cache lifetime settings for redis

[jspeis]

would be nice if users could specify one of:

a) an optional environment variable for a default cache lifetime that would apply to all cubes
b) cache lifetime at the cube level which would override any other cache lifetime setting

if no settings are applied, then the cache may live indefinitely. For simplicity we can use EX arg to specify expiration in seconds: see https://redis.io/commands/set