- move all encryption/decryption, encoding/decoding methods from XrdMq* to the
  SymKeys class so that they are readily available everywhere
- remove any useful methods for encryption from XrdMqMessage since this will go
  away soon
- provide a compatibility layer for OpenSSL 1.0.1 and 1.1.0
- create a proper XrdMgmAuthz object which is an XrdAccAuthorize plugin that will
  be used by the HTTP token access to decide what rights to grant to a user and
  also the check if users coming in with a token have the right to access the
  requested data
- any external authorization plugin (see ALICE use case) will now be loaded in
  mExtAuthz object from the MGM and used for authorizing access
- the default MGM authorization plugin now lives in the libEosMgmOfs.cc library
  and will be used in the future to chain to various types of token authorization
  plugins to enable proper support when used with EOS