def clear_stale_cached_connections!
    keys = @reserved_connections.keys - Thread.list.find_all { |t|
      t.alive?
    }.map { |thread| thread.object_id }
    keys.each do |key|
      checkin @reserved_connections[key]
      @reserved_connections.delete(key)
    end
  end
def clear_stale_cached_connections!
    keys = @reserved_connections.keys - Thread.list.find_all { |t|
      t.alive?
    }.map { |thread| thread.object_id }
    keys.each do |key|
      checkin @reserved_connections[key]
      @reserved_connections.delete(key)
    end
  end
Return any checked-out connections back to the pool by threads that
 are no longer alive.