Replace globals with kong.singletons module variables.

[mars]

Follow-through from mars@f59c849

Replacing globals will not change any functional behavior. It will:

• avoid slow global lookups
• make it easier for devs to see where values come from

The module name singletons was chosen to make it easy to find these anywhere in the project; names like env, environment, config, etc. would lead to confusion.

[mars]

Closing because this code was not cleanly cherry-picked. My local lua env is f*cked and this is broken :(