Fixing Performance Issues with PureComponent

Re-rendering too many components too often can cause performance problems, so it's important to not re-render components that don't need to update - even if their parent re-renders.

React has a PureComponent that we can use to perform a shallow equality check on props and state, which can help with this issue - and prevent unnecessary re-renders.

Summary of Content:

• Force the TodoList component to rerender every 500ms by changing a new counter in state
• Create a new PerfTest component, and log every time that component renders
• Switch PerfTest from React.Component to React.PureComponent to take advantage of the PureComponent shallow equality check on props, and to stop the unnecessary renders