This updates the AdapterCacheRedis instance to be able to handle updating
itself when reading from the cache. For this to work we need to pass a
`fetchData` function to the `get` method.
In the case of a cache miss, we will read the data via the `fetchData`
function, and store it in the cache, before returning the value to the caller.
When coupled with a `refreshAheadFactor` config, we will go a step further and
implement the "Refresh Ahead" caching strategy. What this means is that we will
refresh the contents of the cache in the background, this happens on a cache
read and only once the data in the cache has only a certain percentage of the
TTL left, which is set as a decimal value between 0 and 1.
e.g.
ttl = 100s
refreshAheadFactor = 0.2;
Any read from the cache that happens _after_ 80s will do a background refresh
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