Passing the checkout level to hooks during checkout flow instead of caching

[dparker1005]

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Changes proposed in this Pull Request:

In 3.0, we added caching to pmpro_getLevelAtCheckout() to speed up Add Ons retrieving checkout level information. However, this caching caused issues in edge cases where incorrect level data was retrieved early on page loads.

This PR passes the actual checkout level object to relevant hooks at checkout in lieu of caching the function. This also has the benefit of simplifying Add On code by directly passing checkout levels into functions instead of retrieving them with a function call.

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