Consistent naming of document-client directory to match class DocumentClient

[mzkrasner]

• Updates to generated documentation, package-specific imports, package naming, and directory naming to match DocumentClient

[nathanielc]

I like this but I think we should rename all model-instance-* packages so its consistent. And we should call out in the README description of the package names that document is short for model instance document.

I think only model-instance-handler and model-instance-protocol are left to be renamed.

We should also grep the whole code base for model-instance once this is done to ensure we caught all of them.

[stbrody]

So this is actually going the opposite direction from some changes I had asked Paul to make. I don't really want to call the client something as generic as DocumentClient, because I think it's quite likely that we have multiple streamtypes that operate on JSON documents in the future. Happy to be overridden here, but want to make sure we're thinking about a future where there are multiple json-document based streamtypes with different semantics and how they will all coexist, especially if ModelInstanceDocument is potentially no longer our recommended go-to general purpose JSON document streamtype anymore.