Support for per resource id loaders and sagas

[neurosnap]

Currently an API endpoint in saga-query doesn't support the ability to listen for changes on a per resource id basis. For example:

const fetchUser = api.get('/users/:id');
dispatch(fetchUser({ id: '111' }));
dispatch(fetchUser({ id: '222' }));

In this example, there's no way to have a loader for each individual fetch, we would only be able to know the loading state for fetching a user. In fact, in this scenario the first loader would get clobbered by the second one.

There are other examples where this could be a problem. A user brought up this same issue when trying to create a poll for each resource id here: #10

Another user brought up the same issue here: neurosnap/robodux#55 (comment)

I'm not exactly sure how to handle this but it seems pretty critical that we come up with a scalable solution.

cc @yangm97

[neurosnap]

I came up with a solution that seems to be working relatively well. We create a loader for both ctx.name and ctx.key.

• ctx.name - the name the user provides for the endpoint
• ctx.key - the name + base64 encoded action payload

This provides us with a generic loader for an endpoint as well as a loader specific to the action payload dispatched.