cli: CLI seems to use only synchronous deploys even for sites that are over the "syncfileslimit" and this can lead to upload failures

- Do you want to request a feature or report a bug?

bug

- What is the current behavior?

currently, the CLI always attempts to deploy synchronously, which even for sites with only a few thousand files, can lead to 502 gateway errors during upload, and this causes deploys to fail.

- If the current behavior is a bug, please provide the steps to reproduce.

Try to deploy a site with 10k new files via CLI. It may occasionally work, but often will not. This has been reproduced with sites as small as 4500 files and as big as 60k files. This happens even with high -p X values to set a large timeout.

The setting that was proposed to change that controls this:

https://github.com/netlify/js-client/blob/master/src/deploy/index.js#L25

does not seem to support being changed (or changes are ignored) as we’ve tried setting it to smaller numbers including 1 with the behavior not changing according to our internal logs AFAICT.

- What is the expected behavior?

Deployment works and is handled asynchronously on our servers to allow success no matter how large the site.

- Local Environment Information Happens with v2.37.0 on linux

** Additional netlify-private context** There was a long thread discussing this here in slack: https://netlify.slack.com/archives/C0QFKKY2U/p1582742724079200 . See especially comments from @rybit and @biilmann that we should consider having the CLI use “always async”