The @webapi_request_fields decorator made unnecessary copies of

dictionaries in order to simplify some uncommon lookups, and had two

loops through one of the dictionaries when processing fields. This

wasn't particularly efficient, and made some upcoming work a bit more

difficult.
This change simplifies the logic to operate on existing state, instead

of copies of state, and to simplify the code for both normalizing values

and checking for field requirements.

Djblets and Review Board unit tests pass.