ACME v2 rate limit: too many new orders recently

We have updated to current head (9d43c23af16203b5f94b270fde4b33a77b9cabde), plus a couple of patches, for our production servers, but had to revert after a bit more than two weeks, because certificates for newly added domains weren't getting issued.

The cause for this seems to be this rate limit:

```
{
"type": "urn:ietf:params:acme:error:rateLimited",
"detail": "Error creating new order :: too many new orders recently: see https://letsencrypt.org/docs/rate-limits/",
"status": 429
}
```

which seems to be new to ACME v2. As per [documentation](https://letsencrypt.org/docs/rate-limits/):

> For users of the ACME v2 API you can create a maximum of 300 New Orders per account per 3 hours.

We're certainly not generating that many certificates organically, however, it appears to me that renewals are also counting as an "order". Is anyone able to confirm?

I am thinking this is the case because of the following I pulled from the log. If this is the case - how are renewals for systems with many thousands of domains going to work, if only the first three hundred (renewals) can be handled, and the rest not only fails - but the system is also unable to issue certificates for new organic domains.

This seems like a major issue. I'd appreciate any comment - wasn't following the specifics about ACME v2 too closely (e.g. if it is still possible to make lua-resty-auto-ssl use the v1 API for the time being?). Thanks.

```
+ Checking domain name(s) of existing cert... unchanged.
+ Checking expire date of existing cert...
+ Valid till Jan 18 11:08:23 2020 GMT Certificate will expire
(Less than 30 days). Renewing!
+ Signing domains...
+ Generating private key...
+ Generating signing request...
+ Requesting new certificate order from CA...
err: + ERROR: An error occurred while sending post-request to https://acme-v02.api.letsencrypt.org/acme/new-order (Status 429)

Details:
HTTP/2 429
server: nginx
date: Sat, 28 Dec 2019 05:20:40 GMT
content-type: application/problem+json
content-length: 190
boulder-requester: 73071685
cache-control: public, max-age=0, no-cache
link: ;rel="index"
replay-nonce: 0101Y6s7Fje84XcfGYCVjicPF2XTpFp6Asu6zch1BqnDvFk

{
"type": "urn:ietf:params:acme:error:rateLimited",
"detail": "Error creating new order :: too many new orders recently: see https://letsencrypt.org/docs/rate-limits/",
"status": 429
}
```

No source file, test, or concrete code change is named. Start by tracing how renewal requests reach the ACME v2 /new-order endpoint and compare that behavior with the reported 429 log; done requires confirming the renewal rate-limit behavior and defining an actionable fix for affected deployments.

Written by the indexing model from the issue text.