• Updated README.md with new directives and howto.
• New directives "MDCertificateFile" and "MDCertificateKeyFile" that allow defining
  Managed Domains for certificates coming from somewhere else.
• "MDRenewMode" is the new name of "MDDriveMode" (which is still available for
  backward compatibility). This should make it easier to understand what it does.
• Removing some fields form the store MD JSON, now that we can inspect the pubcert itself
  all the time. Less store updates.
• Dropped support for the pre-v1.x function that mod_ssl used in ancient patches.
• public cert chain now gets cached in memory before server drops privileges, so we always
  have access to it. Allowed to drop some pre-computed values such as the SHA256 fingerprint.
• Adding "" as a shortcut to "". It just reads that much better.
• More "how to"s in README.md
• MDPortMap directive now also accepts http and https as external ports. http:8888 would
  mean that http requests from the internet arrive on local port 8888.
• Started "how to"s in README.md
• MDRequireHttps handler now runs after mod_ssl, so that it can report SSL errors before.
• All GET requests against an ACMEv2 endpoint (except directory and nonce retrievals), are
  now made as POST with an empty, JWS signed body. See
  https://community.letsencrypt.org/t/acme-v2-scheduled-deprecation-of-unauthenticated-resource-gets/74380
  for the necessity of this API change at LetsEncrypt.