Safeguard against already existing _acme-challenge records

[Tea23]

I noticed that sometimes my renewal would fail, either because of a timeout or something else. In these cases, the _acme-challenge record wouldn't be cleaned up and then any future runs of the renewal would then fail because the record already exists (ERROR: Response: {"result":null,"success":false,"errors":[{"code":81057,"message":"Record already exists."}],"messages":[]})

This adds a simple check to see if the record already exists, and deletes it if it does. I considered trying to clean up after failure instead, but this actually catches other potential sources of _acme-challenge, such as if multiple tools are used.

[GrantGryczan]

I'm not a fan of cleaning up other potential sources of _acme-challenge. Cleaning up strictly the records this script created would be ideal, because otherwise you could be deleting _acme-challenge records that are still in use. The DNS challenge takes half a minute, so it's not unlikely that two of these on the same domain starting around the same time (which I have) would conflict with each other.

Also, cleaning up immediately after failure wouldn't be robust since a system reboot (or taking down a container, if it's in a container) would prevent a post-failure cleanup from running. I think a better way might be to save a temporary file remembering which _acme-challenge record will be created before creating it, and delete that file after a successful run. Then, if the file exists when the script starts, delete the record it refers to.

[socram8888]

I cannot accept the PR as is - code should use tabs as the rest, and it should cache the result of list_record_id into a variable rather than calling twice.