NodeRequest member function instead of v8::Context

[mikemorris]

Resurrects #5527, but with a JavaScript shim in front to preserve the callback-passing public API.

/cc @tmpsantos @jfirebaugh @miccolis

[mention-bot]

@mikemorris, thanks for your PR! By analyzing the annotation information on this pull request, we identified @kkaefer, @lucaswoj and @jfirebaugh to be potential reviewers

[jfirebaugh]

To bulletproof this for production, we should probably tweak the shim so that the exported Map.prototype has all the same properties as before, in case somebody is using Map.prototype.load.call(map, ...) or something like that.

I haven't actually tested it, but the following might work:

var Map = mbgl.Map;
var originalConstructor = Map.prototype.constructor;
Map.prototype.constructor = function(options) {
    ...
}

[mikemorris]

Rebased this and updated the shim to export the proper prototype, added tests (which were passing on master, failing here before but fixed now) to confirm expected behavior.