so that it has fetched the most from the master's binlog before the master
accidently dies. SCRUM task #756.


sql/slave.cc:
  Replication: give the slave I/O thread a higher priority than the slave SQL thread
  so that it has fetched the most from the master's binlog before the master
  accidently dies.
sql/slave.h:
  Replication: give the slave I/O thread a higher priority than the slave SQL thread
  so that it has fetched the most from the master's binlog before the master
  accidently dies.