Author: sam marshall
MDL-72328 cachestore_redis: Add TTL support for Redis cache
A list of times for each cache key in a TTL cache is kept in a Redis
sorted list, which can be queried efficiently to delete expired
cache items later in a scheduled task.
This change makes set and delete 2x slower (only for caches which use
TTL) but there is no impact on get performance.
+ 4.0 en [task_ttl,cachestore_redis]
+ 4.0 en [test_ttl,cachestore_redis]
+ 4.0 en [test_ttl_desc,cachestore_redis]