Fix 'aborted' detection on Node v15.5.0+

This fixes a major memory leak I encountered usign this on Node v16. Doing a binary search of node versions, the problem appears to originate in v15.5.0 as a result of nodejs/node#33035, however later changes have quietly completely removed the 'aborted' event that http-proxy relies on, and later added a deprecation note about it (which seems to actually be incorrect). Despite what the notes about DEP0156 say, the only way I could get this module working reliably again was to replace req.on('aborted') with instead checking res.on('close') and looking at res.writeableFinished.

For a very easy test, load any video file over the proxy, and seek backwards in Chrome. For example, run the following:

const httpProxy = require('http-proxy');

httpProxy.createProxyServer({
  target: 'http://files.dashingstrike.com.s3.amazonaws.com',
  changeOrigin: true,
}).listen(8080);

And then open http://localhost:8080/SplodyGameplayLong.mp4, and then seek backwards in the file. Chrome will abort hundreds of requests (each of which is requesting ~32MB), which, if not properly aborted, will leak connections as well as hundreds of MBs of TCP kernel memory per second (quickly bricking your instance, making it unreachable even by SSH, as I found out -_-). On Linux, you can view TCP kernel memory used with cat /proc/net/sockstat (mem is count of 4K pages), or ss -atn state big if you have ss installed.