• When the server is not started by root, the module no longer tries to chown() the
  staging and challenges directories as User and Group directives are ignored. Fixes #115.
• When a md.json file in the store cannot be parsed, it is ignore and overwritten during
  initial synch at server start.
• When migrating from ACMEv1 to ACMEv2 on the same host, the account private key of an
  existing account is reused on registration. Let's Encrypt internally treats this as
  the same account, although it is exposed using different urls. This helps keeping
  the waste of large prime numbers at a manageable level and allows for more useful
  statistics from ACME CAs.