Fixes #52 - Ensure key/cert are in a consistent state despite errors

[zsteinkamp]

Proposed changes

Issue #52 details a scenario where a failure during ACME validation can leave the target system in a failed state where HTTPS does not work at all. The nature of the problem is that the new .key file is written before the ACME validation step runs. If that validation fails, then the system is left with a new .key file and an old .crt file, which is invalid.

This change is to first write the new .key to a .key.tmp file, do the ACME validation, store the .crt, then rename the .key.tmp file to .key before purging the cache.

Checklist

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• [x] I have read the CONTRIBUTING document
• [x] If applicable, I have added tests that prove my fix is effective or that my feature works
• [x] If applicable, I have checked that any relevant tests pass after adding my changes
• [x] I have updated any relevant documentation (README.md and CHANGELOG.md)