The update log itself was removed back in 5.0. Recommit
with post-review fixes.


sql/log.cc:
  OPTION_UPDATE_LOG was set in all threads but replication ones.
  So, it seems that the check filtered out slow log records from
  replication threads. Now we do it with explicit check.
sql/mysql_priv.h:
  remove unused define
sql/mysqld.cc:
  Do not set OPTION_UPDATE_LOG. It is not used anymore.
sql/set_var.cc:
  We never check for OPTION_UPDATE_LOG. So, we should not bother setting it.