stream: Change default highWaterMark for objectMode to 16 instead of 16384

Currently streams with `objectMode:true` share the same default highWaterMark as regular streams.
This means that they will buffer up to 16384 objects which seams quite excessive.

``` js
var rs = new (require('stream').Readable)({objectMode:true});
var buffered = 0;
rs._read = function() {
  console.log('buffered:', buffered++, 'objects');
  rs.push({hello:'world'});
};
rs.read(); // buffers 16384 objects (~25mb of ram on my machine)
```

This PR changes the default highWaterMark when using objectMode to 16 which seams more reasonable.