After MDEV-32551, in a master/slave setup, if the replica's IO thread
quickly and successively reconnects (i.e quickly running
STOP SLAVE IO_THREAD followed by START SLAVE IO_THREAD), the relay log
rotation behavior changes. That is, MDEV-32551 changed the logic of the
binlog_dump_thread on the primary, such that it can stop itself before
sending any events if it sees a new connection has been created to a
replica with the same server_id. Pre MDEV-32551, the connection would
establish and it would send a "fake" rotate event to populate the
log name. Post MDEV-32551, the connection stops itself, and a rotate
event is not sent.

This made the test rpl.rpl_mariadb_slave_capability unstable because
it is reliant on the name of the relay logs (which is dependent on the
number of rotates); and the pre-amble of the test would quickly
start/stop the IO thread. There a binlog dump thread could end itself
before sending a rotate event to the replica, thereby changing the name
of the relay log.

This patch fixes this by adding in a synchronization in-between IO thread
restarts, such that it waits for the primary's binlog dump threads to
sync up with the state of the replica.