zlib: simplify flushing mechanism

Previously, flushing on zlib streams was implemented through
stream 'drain' handlers. This has a number of downsides; in
particular, it is complex, and could lead to unpredictable
behaviour, since it meant that in a sequence like

```js
compressor.write('abc');
compressor.flush();
waitForMoreDataAsynchronously(() => {
  compressor.write('def');
});
```

it was not fully deterministic whether the flush happens after
the second chunk is written or the first one.

This commit replaces this mechanism by one that piggy-backs
along the stream’s write queue, using a “special” `Buffer`
instance that signals that a flush is currently due.

PR-URL: https://github.com/nodejs/node/pull/23186


Reviewed-By: James M Snell <jasnell@gmail.com>