MDL-72837 core_cache: Add versioned cache support

Adds new set_versioned and get_versioned APIs to cache, which means you can
request a specific version from cache and it will not return an outdated
version.

This is important when using multi-layer-caches where a local cache might have
an outdated version of the cache, but the shared cache has a current version.
With this feature, the content of the cache does not have to be rebuilt, as
it will automatically retrieve it from the shared cache if necessary.

cache/lib.php

@@ -225,4 +225,4 @@ class cacheable_object_array extends ArrayObject implements cacheable_object {
         $class = __CLASS__;
         return new $class($result);
     }
-}
+}