When the transaction isolation level is SERIALIZABLE, or when
a locking read is performed in the REPEATABLE READ isolation level,
InnoDB must lock delete-marked records in order to prevent another
transaction from inserting something.
However, at READ UNCOMMITTED or READ COMMITTED isolation level or
when the parameter innodb_locks_unsafe_for_binlog is set, the
repeatability of the reads does not matter, and there is no need
to lock any records.
row_search_mvcc(): Skip locks on delete-marked committed records upfront,
instead of invoking row_unlock_for_mysql() afterwards. The unlocking
never worked for secondary index records.
