Bug#41756 "Strange error messages about locks from InnoDB".

In JT_EQ_REF (join_read_key()) access method,
don't try to unlock rows in the handler, unless certain that
a) they were locked
b) they are not used.

Unlocking of rows is done by the logic of the nested join loop,
and is unaware of the possible caching that the access method may
have. This could lead to double unlocking, when a row
was unlocked first after reading into the cache, and then
when taken from cache, as well as to unlocking of rows which
were actually used (but taken from cache).

Delegate part of the unlocking logic to the access method,
and in JT_EQ_REF count how many times a record was actually
used in the join. Unlock it only if it's usage count is 0.

Implemented review comments.


mysql-test/r/bug41756.result:
  Add result file (Bug#41756)
mysql-test/t/bug41756-master.opt:
  Use --innodb-locks-unsafe-for-binlog, as in 5.0 just
  using read_committed isolation is not sufficient to 
  reproduce the bug.
mysql-test/t/bug41756.test:
  Add a test file (Bug#41756)
sql/item_subselect.cc:
  Complete struct READ_RECORD initialization with a new
  member to unlock records.
sql/records.cc:
  Extend READ_RECORD API with a method to unlock read records.
sql/sql_select.cc:
  In JT_EQ_REF (join_read_key()) access method,
  don't try to unlock rows in the handler, unless certain that
  a) they were locked
  b) they are not used.
sql/sql_select.h:
  Add members to TABLE_REF to count TABLE_REF buffer usage count.
sql/structs.h:
  Update declarations.