REFACTOR Purchasable - setIsAvailable

[jsirish]

cache $is_available

[jsirish]

So this works great when testing the FE and CMS, and saves 4 queries on page load.

However, it consistently fails PurchasableTest at the following point:
https://github.com/dynamic/silverstripe-foxy/blob/master/tests/Extension/PurchasableTest.php#L113

The test passes if instead of returning $this->is_available I return $this->setIsAvailable() and just return the value of $available vs. setting the static