Fix replication for multi-update
new test - rpl_multi_update2
sql/mysql_priv.h:
Bug#7011
New function mysql_multi_update_lock()
sql/sql_parse.cc:
Bug#7011
New function check_multi_update_lock()
For multi-update on slave, perform an early open&lock
sql/sql_update.cc:
Bug#7011
Split out multi-update locking into its own function,
mysql_multi_update_lock()
