The primary purpose of Cache\CacheItemPoolInterface is to accept a key from
the Calling Library and return the associated Cache\CacheItemInterface object.
It is also the primary point of interaction with the entire cache collection.
All configuration and initialization of the Pool is left up to an
returnarray|\Traversable A traversable collection of Cache Items keyed by the cache keys of
each item. A Cache item will be returned for each key, even if that
key is not found. However, if no keys are specified then an empty
traversable MUST be returned instead.
Confirms if the cache contains specified cache item.
Note: This method MAY avoid retrieving the cached value for performance reasons.
This could result in a race condition with CacheItemInterface::get(). To avoid
such situation use CacheItemInterface::isHit() instead.