perf: js plugin should start from now and work out to max clock skew

[asoorm]

  for (var i = now - ALLOWED_CLOCK_SKEW; i <= now + ALLOWED_CLOCK_SKEW; i ++) {

    var raw = token + SHARED_SECRET + i;

    var shaObj = new jsSHA("SHA-256", "TEXT");
    shaObj.update(raw);
    var hash = shaObj.getHash("HEX");

    if (hash === signature) {
      return preBodyToHeaders.ReturnData(req, sess.meta_data);
    }
  }

Current implementation checks from now - ALLOWED_CLOCK_SKEW to now + ALLOWED_CLOCK_SKEW.

More appropriate would be to check from now, and work out to avoid needless hash checking.