MDL-40538 caching: Use isset for performance
isset is much faster than array_key_exists. For caches we need to
ensure as much performance as possible. To all null to continue to
be stored and provide correct results, non ttl also now saves in
$this->store[$key][0]. As isset($this->store[$key]) will return
true in all cases.
time() as found in cache::now() can be slow on some platforms and
we shouldn't bother checking it unless needed. It also stops non-ttl
caches paying the ttl price.
Unit tests were adjusted to ensure null is still handled correctly
when we are only using isset.
