Description

  -----------

  Originally Semisync use the SELECT() function to listener all slave sockets,

  But SELECT() has several restrictions, so replace it by poll();

  Ack_receiver maybe read invalid socket fd and report network packet errors.

  We should clear socket when read return error with ER_NET_READ_ERROR, if not,

  Ack_receiver will report a lot of network packet errors until dump thread is

  killed;