returns too many rows

Patch #12 - Clean up "materialized" state in JOIN_TAB vs TABLE_LIST.

The patch is based on assuming that a materializable derived table
that is determined to be const (contains maximum one row), is always
materialized by the optimizer before activating the executor.
A new functiom materializable_is_const() is implemented for
materializable tables that determines whether this is the case, and
the join_materialize_derived() function check whether such table
is const before actually materializing it.

sql/records.h
  Deleted unused fields copy_field and copy_field_end from READ_RECORD.

sql/sql_derived.cc
  Eliminate use of TABLE_LIST::materialized. Instead use
  TABLE_LIST::materializable_is_const() to determine whether to
  materialize a derived table that is const.

sql/sql_executor.cc
  In join_materialize_derived(), assume that a derived table that is
  denoted "const" is already materialized.

sql/sql_optimizer.cc
  In get_quick_record_count(), use estimated rowcount for a derived table
  that is const. (However, this code is never invoked by the test suite).

sql/table.cc
  Implement the function TABLE_LIST::TABLE_LIST::materializable_is_const().
  It should be called only for a materializable function 
  (uses_materialization() is TRUE), and it uses the row count estimate
  to determine whether the table is assumed to be const.
  In TABLE_LIST::fetch_number_of_rows(), safely use the estimated row count
  also for a materializable table that is const.

sql/table.h
  Define function materializable_is_const(), and remove the "materialized"
  field from struct TABLE_LIST.