When receiving no events from its master longer than slave_net_timeout
slave's replication io thread disconnects and reconnects. Reconnecting
causes rotation of the relay log. That is unnecessary work which also
causes inconvenience because of changes of the relay log files names:
old files are removed, new are created.
  
  Fixed with introducing the Heartbeat event new functionality.
  Heartbeat event is generated in master's idle time by dump thread.
  Frequency of sending the event is optional and is determined by slave.
  The optimal value is within [0.001, slave_net_timeout] interval.
  
  The project introduces master_heartbeat_period option for CHANGE
  MASTER sql clause.  The requested on the slave side value for the
  period is passed to the dump thread on the master side.  The dump
  thread sends a heartbeat replication event if there is no more
  unsent events in the actual binlog file for a period longer that
  master_heartbeat_period.  Whenever the master's binlog is updated
  with an event, the waiting for heartbeat sending condition gets
  reset.
  
  Heartbeating is requested implicitly with the period
  slave_net_timeout/2 when no master_heartbeat_option was
  provided. if the option's value is set explicitly zero there will
  be no heartbeats.
  slave_net_timeout is updated now with generating a warning if the
  new value is less than the current heartbeat period.

  Two status variables on the slave side allows to monitor
  heartbeats flow.
  
  The test checks the syntax for the new option, the valid range -
  errors and warnings on reasonable values; the fact that there is no
  relay log rotation (thereafter no reconnection) while more than
  slave_net_timeout seconds elapsed and the master has been idling;
  new status variables.