Implement CRM_Utils_Cache_Tiered

Implement CRM_Utils_Cache_Tiered

Before
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* No way to daisy-chain caches to form a cache hierarchy
* ArrayCache::reobjectify() would fail to reobjectify objects in an array, which seems against the spirit of PSR-16

After
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* You can create a cache hierarchy with `new CRM_Utils_Cache_Tiered([$fastCache, $mediumCache, $slowCache])`
* ArrayCache::reobjectify() will reobjectify if it detects an object directly in an array

Note: To ensure that TTL data is respected consistently regardless of how
the tiers behave and the order in which they are used, the TTL/expiration
must be stored extra times.