ref: convert req.hashedFields to Set

[mantariksh]

Problem

myinfo.server.controller assigns a hashedFields key to the Express request object in order for subsequent middlewares to keep track of which MyInfo attributes have been verified through their hashes. hashedFields is an object which maps the MyInfo attribute to the corresponding client response, but subsequent middlewares have no need for the values of this object, since they only check that the value corresponding to each key is truthy, i.e. that the object contains the key.

Solution

Convert hashedFields into a Set, which is the more correct data structure for this case. This ensures that subsequent refactors (#560) don't have to bend over backwards just to get hashedFields into the right format.

Also, assign hashedFields to res.locals instead of req.