Cancel pending style request when loading style JSON

[louwers]

#3240 added a new method to load a style JSON from string. However when a map is loading a style asynchronously, this can lead to a race condition where the style set from JSON is overwritten by the incoming style response.

Includes a regression test.

Closes #3708

[TimSylvester]

That looks good.

Using the destructor to potentially do network stuff makes me nervous, but that's another matter.

[louwers]

@TimSylvester Nice. That destructor deletes the FileSourceRequest, which calls the cancellation callback. I don't think that actually cancels in-flight requests so it's not really doing network stuff per se. It also deletes the mailbox. So if there's a network request coming in, it will just send a message to a dead mailbox.