Allow sharing of cached smugmug session

Right now the cache table stores a session id, but the way it is keyed it stores it separately for smugmug image and smugmug video provider.

Two possibilities to fix this.
1. If possible, change the "provider" element of the arghash to something generic and shared between smugmug providers
2. Add code logic to the get session function to check for valid sessions from other smugmug providers and use those if available first