feature request - read/write test on startup

[jvanasco]

Debugging this took way too long. I recently upgraded a server and all my certificate renewals started to fail. I eventually found in the error messages:

Jun 26 20:55:12 westing acme-dns[1802008]: time="2020-06-26T20:55:12Z" level=debug msg="Error while trying to update record" error="attempt to write a readonly database"

It would be nice if acme-dns tried to read&write on startup, to ensure it can do both - and print out a warning or exit if it can't.

--

Note for others: The cause of my issue was somewhat related to #88 - my database file and parent-directory were owned by acme-dns... but the parent-directory was actually a symlink to another directory on a mounted volume; the symlink location was owned by acme-dns but the real location was not.