Bug fixes: robust release of media streams + memoization of audio/video constraints

[Tiberiu02]

First, due to the getMediaStream function being asynchronous, streams might not be released properly by the cleanup function of the main useEffect. This leads to media streams that keep running even after the ReactMediaRecorder is unmounted, causing performance and privacy issues. The new architecture ensures that there is only one media stream running at every time and stops it when the component unmounts.

Second, the video and audio media constraints are passed to the useEffect depedency list as objects, but the React dependency list compares objects by reference, not value. If the video and audio media constaints objects are created inside the same component that is calling the hook, their reference will be different every time. This would cause the useEffect to be called on every render, resulting in infinite re-renderings. This PR also fixes that by memoizing the video and audio constaints objects.