Add two different methods to remove listeners for read and write events.
Add two different methods to remove listeners for read and write events.

Those methods are not yet part of React\EventLoop\LoopInterface, we need
to fix the current implementation of the libevent-based loop class before
"promoting" them.

This commit also fixes the iteration on callbacks registered for write events.

krageon/react,cystbear/react,FireWalkerX/react,nemurici/react,dagopoot/react,sachintaware/react,sachintaware/react,cystbear/react,reactphp/react,nemurici/react,ympons/react,FireWalkerX/react,krageon/react,naroga/react,ericariyanto/react,dagopoot/react,cesarmarinhorj/react,ericariyanto/react,naroga/react,cesarmarinhorj/react,ympons/react