support flush on demand for node.js client

[kim5566]

hey team,

i'm wondering if node.js client could support flush on demand like the classic one does before,

analytics.flush(function(err, batch){
  console.log('Flushed, and now this program can exit!');
});

instead of call .closeAndFlush() to stop collecting new events.

The use case i have is, i have set flushAt, for example, 100 and let batch processing events.

There's one particular time sensitive event, when it fired, i want to send immediately instead of waiting for the batch.

But i still want to keep collecting new events after flush.

[silesky]

Thanks @kim5566 for taking out an issue, we are looking at bringing this back -- see #822

[silesky]

Tracking this: #822 -- closed for consolidation purposes.