docs: function cacheNewKeyPairs () {
          if(pregeneratedKeyPairs.length < desiredCacheLength && !pending) {
              pending = true;
              generateKeyPair(function (err, pemKeyPair) {
                  pending = false;
                  pregeneratedKeyPairs.push(pemKeyPair);
                  cacheNewKeyPairs();
              });
          }
      }