docs: function verifyExpiration() {
      if(opt.removeExpired === false) return;
      let items = thorin.util.readDirectory(rootPath, {
        ext: '.sess'
      });
      let calls = [],
        removes = 0,
        now = Date.now();
      items.forEach((fpath) => {
        calls.push((done) => {
          fse.readJson(fpath, (e, json) => {
            if(e) {
              logger.warn('Failed to lookup session file [%s] for cleaning.', fpath, e);
              return done();
            }
            if(now < json.e) return done();
            // remove the file.
            fse.remove(fpath, (e) => {
              if(e && e.code !== 'ENOENT') {
                logger.warn(`Failed to remove expired session file ${fpath}.`, e);
              } else {
                removes++;
              }
              return done();
            });
          });
        });
      });
      async.series(calls, () => {
        if(removes > 0) {
          logger.trace(`Removed ${removes} expired sessions.`);
        }
        setTimeout(verifyExpiration, CLEANUP_TIMER);
      });
    }