fixed a couple of bugs with SEQ_READ_APPEND cache
rpl000016 still has non-deterministic result, but I am going to commit and
push since what I have is now better than what is in the main repository


client/mysqltest.c:
  added wait_for_slave_to_stop
  cleaned up TODO and comments
include/my_sys.h:
  fixed race in flush_io_cache in SEQ_READ_APPEND cache
mysql-test/r/rpl000016.result:
  updated result
mysql-test/t/rpl000016.test:
  use wait_for_slave_to_stop to have deterministic slave behaviour for the test
mysys/mf_iocache.c:
  fixed race in flush_io_cache()
  fixed failure to unlock mutex in my_b_append()
sql/log.cc:
  be compatible with 3.23 master
sql/log_event.cc:
  3.23 master compat
sql/slave.cc:
  3.23 master compat
sql/sql_class.h:
  compat with 3.23 master