The semisync ack collector hits fd's out-of-bound value assert through
a stack of

/usr/sbin/mysqld(_ZN12Ack_receiver17get_slave_socketsEP6fd_setPj+0x70)[0x7fa3bbe27400]

/usr/sbin/mysqld(_ZN12Ack_receiver3runEv+0x540)[0x7fa3bbe27980]

/usr/sbin/mysqld(ack_receive_handler+0x19)[0x7fa3bbe27a79]

The reason of the failure must be the same as in https://bugs.mysql.com/bug.php?id=79865
whose fixes are applied with minor changes.

Specifically, the semisync ack thread is changed to use poll()
instead of select() on platforms where the former is defined.
On the systems that still use select(), Ack receive thread will generate
an error and semi sync will be switched off. Windows systems is exception
case because on windows this limitation does not exists.

The sustain manual testing with `mysqlslap --concurrency > 1024' in "background" while the slave io thread is restarting multiple times.