fix: logEvent callback on content block - #329

Hey @donaldpipowitch, thanks for this PR. Were you able this working on a test app?

I tried these changes out with create-react-app + ublock but the callbacks still weren't being called

import logo from './logo.svg';
import './App.css';
import amplitude from 'amplitude-js'
import React, { useEffect } from 'react';

const callback = (responseCode responseBody) => {
  console.log(responseCode)
  console.log(responseBody)
}

function App() {
  useEffect(() => {
    amplitude.getInstance().init("MY_API_KEY")
    amplitude.getInstance().logEvent('Amplitude-JavaScript-Demo: initialized', {}, callback)
  });

  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <button onClick={handleClick}>Send Event</button>
      </header>
    </div>
  );
}

export default App;

My bad... of course sending the request is still async. So this is triggered by _sendEventsIfReady which gets the callback even though _sendEventsIfReady doesn't expect a param. Strange.
But the callback is kept inside _unsentEvents and _sendEventsIfReady triggers sendEvents... which really makes a request - and there is no error handling. And it looks like I need to call removeEvents in order to invoke the previously saved callback. So I guess what I really need to fix is:

@donaldpipowitch I looked deeper into the fix. It belongs in _limitEventsQueued and I worked off your branch in #342. Thanks for bringing the issue up!