code: function(pool) {
      let toRemove = Math.max(1, pool.idleConnections() - opts.minimumIdle);
      while (toRemove > 0) {
        const conn = idleConnections.peek();
        --toRemove;
        if (conn && conn.lastUse + opts.idleTimeout * 1000 < Date.now()) {
          idleConnections.shift();
          conn.forceEnd().catch(err => {});
          conn.releaseWithoutError();
          continue;
        }
        break;
      }
      ensurePoolSize(pool);
    }
docs: Permit to remove idle connection if unused for some time.
  @param pool  current pool