code: function post_token(oidc_token) {
      return new Promise((resolve, reject) => {
          dbMgr
              .updateOne("oidc_token", {
                  id_token: {
                      $eq: null
                  }
              }, {
                  $currentDate: {
                      expire_on: true
                  },
                  $set: oidc_token
              }, {upsert: true})
              .then((result) => {
                  if (result.upsertedCount === 1) {
                      return resolve(result.upsertedId._id);
                  } else {
                      return reject(new Error("No oidc_token recoreds inserted"));
                  }
              }, (err) => {
                  reject(err);
              });
      });
  }
docs: This function must return a promise, which persists the oidc_token
  object, and resolve with the ID for the persisted oidc_token.  The
  ID can be used later to retrieve the persisted oidc_token.
  
  @param   oidc_token
  @return oidc_token_id