`changesReader` doesn't batch

## Expected Behavior

I would expect the following code to wait 10 seconds and then return a batch of messages from the last position in the changes feed. So if I updated the DB 5 times within that 10 second timeout I would expect to see: "a batch of 5 changes has arrived" after 10 seconds.

messages.changesReader
.start({ timeout: 10000 })
.on('batch', (batch) => {
console.log('a batch of', batch.length, 'changes has arrived');
}).on('error', (e) => {
console.error('error', e);
});

## Current Behavior

What is happening is that no batching is taking place. Every time I update the database within that timeout I _immediately_ get the log "a batch of 1 changes has arrived".

Have I missed something obvious or misunderstood how nano's batching works?

## Your Environment

* Node v16.5
* npm 8.5.5
* Windows 10

Start by reproducing the supplied Node.js changesReader example with the stated timeout and update sequence. Trace the changesReader entry point to determine why each update emits a single-item batch; done means updates within the timeout produce one batch after the timeout, with errors still reported through the shown handler.

Written by the indexing model from the issue text.