fix loading states observables

[jondayton]

We have a loadingStates property in the datastore, which is an observable Map object keyed by a query tag, and with values of sets of query information that is added and removed as fetches start / complete.

Previously, the Map and Set were initialized separately, with the Set initialized as a shallow observable so that its objects would not be observable, and thus would not be proxied objects and could be compared directly.

Map() // observable
  => Set() // observable
    => Object() // not observable

However, because mobx propagates observability down, this seems to have broken after a recent change in a mobx Typescript interceptor

This PR changes the data structure to initialize observability once, and change the query reference objects to JSON strings so that they will not be proxied themselves.

Map() // observable
  => Set() // observable (by extension)
    => JSON String() // not observable

[jackiekircher]

looks like this redundant line got left in

[jondayton]

removed