Feature: Access cached items within TagDependency

[xepozz]

Using TagDependency as a cache item dependency does not provide ability to access all tagged cache items.

But it could be done with another TrackedTagDependency wrapper, which will remember what keys set while doing $cache->set()

So in total TrackedTagDependency should contain:

• timestamp (cache reset time)
• keys

keys could be serialized or drivers may use suitable data structures to store the keys internally, some kind of tables

Use cases for the TrackedTagDependency:

• use different points of cache with the same cache tag
• fetch all these cache keys (not values) to decide how they might be interpreted
• possible to get the values by $cache->multiGet() method

Alternative solution is to track the keys in each place of $cache->set():
$cache->set($key)
$cache->add($tagName, [$key]) – add $key to a table

Implementation is under discussion.
Also it could be possibly applied to Yii3 Cache package