temptable views

The TABLE::key_read field indicates if the optimizer has found that row
retrieval only should access the index tree. The triggered assert
inside close_thread_table() checks that this field has been reset when
the table is about to be closed.

During normal execution, these fields are reset right before tables are
closed at the end of mysql_execute_command(). But in the case of errors,
tables are closed earlier. The patch for Bug#52044 refactored the open
tables code so that close_thread_tables() is called immediately if
opening of tables fails. At this point in the execution, it could
happend that all TABLE::key_read fields had not been properly reset,
therefore triggering the assert.

The problematic statement in this case was EXPLAIN where the query
accessed two derived tables and where the first derived table was
processed successfully while the second derived table was not.
Since it was an EXPLAIN, TABLE::key_read fields were not reset after
successful derived table processing since the state needs to be 
accessible afterwards. When processing of the second derived table
failed, it's corresponding SELECT_LEX_UNIT was cleaned, which caused
it's TABLE::key_read fields to be reset. Since processing failed,
the error path of open_and_lock_tables() was entered and
close_thread_tables() was called. The assert was then triggered due
to the TABLE::key_read fields set during processing of the first
derived table.

This patch fixes the problem by adding a new derived table processor,
mysql_derived_cleanup() that is called after mysql_derived_filling().
It causes cleanup of all SELECT_LEX_UNITs to be called, resetting
all relevant TABLE::key_read fields.

Test case added to derived.test.