fix(analytics-browser): should send batches sequentially

[Mercy811]

Summary

AMP-118975

Fix the issue that events are not received by order by the server. The root cause is that Promise.all() executes calls concurrently thus resolve order is not guaranteed. Fix it by sending batches sequentially, in other words, only send the next batch when the preceding one resolves.

See comment for additional notes.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?:

[Mercy811]

I just noticed that it's auto changed by Intellij

I had

await batches.reduce(
      (promise, batch) => {
        return promise.then(() => this.send(batch, useRetry))
      },
      Promise.resolve()
    );

After clicking that "convert to async function", it updated to what we have here.

I was expecting to have

await batches.reduce(
      async (promise, batch) => {
        await promise;
        return this.send(batch, false);
      },
      Promise.resolve()
    );

It's still working although a little bit redundant. Should clean it in AMP-120671.