ChangeSet 1.1620.12.1 and ChangeSet 1.1625.2.1
from 4.1. This makes the slave I/O thread flush the relay log 
after every event, which provides additional safety in case
of brutal crash (reduces chances to lose a part of the relay log).


sql/repl_failsafe.cc:
  update for new prototype
sql/slave.cc:
  The slave I/O thread now flushes the relay log after writing
  every event to it, like we already do in 4.1.
sql/slave.h:
  new prototype
sql/sql_repl.cc:
  update for new prototype