DO NOT MERGE: Multithreaded javascript

[darind]

In this PR we are introducing the enableMultithreadedJavascript feature flag which allows to execute javascript code from arbitrary threads.

Consider the following example:

new java.lang.Thread(new java.lang.Runnable({
    run() {
        const mainLooper = android.os.Looper.getMainLooper();
        const currentLooper = android.os.Looper.myLooper();
        const isMainThread = currentLooper === mainLooper;
        console.log(isMainThread); // should print false
    }
})).start();

We have identified 3 tasks to implement the feature:

• Introduce v8::Locker objects to ensure re-entrancy of the V8 isolates
• Thread-safe JNI
• Runtime.currentRuntime should not rely on ThreadLocal storage only. When running on a background thread it should retrieve the runtime from the currently entered V8 isolate.

[saschaarthur]

whats the progress on this here?
Looks like a nice Feature..

[darind]

This PR was created as a POC to demonstrate the technical feasibility of this concept. Enabling it would break existing plugins and we are not planning on doing that at the moment. You may try it out in your application and report if you are experiencing any issues or performance improvements.

[farfromrefug]

@darind @NathanaelA Seeing this is not enabled by default. Could we merge it ? A user would have to know what he is doing as it would mean enabling in the package.json.

[farfromrefug]

@NathanaelA agree about tests! About enabled by default in 7.0 i would be careful, we would need to test it in multiple and different production apps . As Darin said plugins would need to handle that correctly.

[triniwiz]

It can be on by default for 8.x that should give plugin authors some time to update their plugins