JS-41: make API objects immutable

[RangerRick]

This PR updates a number of the API objects to be immutable, and reworks the API for interacting with the client and HTTP options and such a little bit as a result.

It also includes a refactor of the mock HTTP implementation(s) as they had kind of become a mess and I wanted to tweak them as I was testing DAO changes.

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[mattixtech]

I only skimmed the PR so far, but I'd like to know your thoughts on switching to builder pattern to achieve immutability instead of using the with factory methods. Will review further once we resolve that decision.

[RangerRick]

FYI, the force-pushes were just some other cleanup I ran into while testing, but the other new commits are relevant to your feedback, @mattixtech

[RangerRick]

I thiiiink this takes care of all your feedback, @mattixtech. Lemme know how it looks.