[core] Clean up FileSource

[jfirebaugh]

👀 @tmpsantos @kkaefer @mikemorris

Clean up FileSource by taking advantage of the safe callback and cancellation semantics provided by RunLoop::invokeWithCallback.

Fixes #2827, eliminates the complex logic around Resource destruction, eliminates ResourceHolder, moves us toward #2909.

Also allows simplification of the node implementation of FileSource: we can use RunLoop to handle dispatch back to the node event loop. I removed JS-level cancellation via the optional "cancel" property; it was untested and unused. (Note that RunLoop::invokeWithCallback handles cancellation at the C++ level; i.e. not calling the callback if the request has already been cancelled.)

[tmpsantos]

Good catch.

[tmpsantos]

Kudos @jfirebaugh, I really liked this.

[mikemorris]

Aside from the following minor naming issue this is working great for me, and fixes the segfaults I was seeing previously in #2334.

../../src/mbgl/storage/default_file_source.cpp: In member function ‘void mbgl::DefaultFileSource::cancel(const mbgl::Resource&, mbgl::FileRequest*)’:
../../src/mbgl/storage/default_file_source.cpp:74:78: error: declaration of ‘request’ shadows a member of 'this' [-Werror=shadow]
 void DefaultFileSource::cancel(const Resource& resource, FileRequest* request) {
                                                                              ^

@miccolis Is removing the JavaScript cancel interface an issue at the implementation level?

[miccolis]

@mikemorris removing cancel is fine.