MDEV-5446: Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' fails on EXPLAIN EXTENDED with VALUES function

field_index should be set correctly for null field created by Item_insert_value::fix_fields().

(as it's done in 10.0)

Item_default_value::arg can be NULL so walk() should take it into consideration.

Item_default_value::arg can be NULL so walk() should take it into consideration.

MDEV-5740: Assertion `!derived->first_select()->exclude_from_table_unique_test || derived->outer_select()-> exclude_from_table_unique_test' failed on 2nd execution of PS with derived_merge

Do not check tables of executed units.
Debug info about stages of derived tables execution added.

The calls of the function remove_eq_conds() may change the and/or structure
of the where conditions. So JOIN::equal_cond should be updated for non-recursive
calls of remove_eq_conds(). 

(assert added by MySQL verified that strnxfrm can only *increase*
the string length if from == to, and the latter is a random
decision made by individual items and String::realloc).

Fix it by avoiding the memcpy in the first place.

Reason for the bug was an optimization for higher connect speed where we moved when global status was updated,
but forgot to update states when slave thread dies.
Fixed by adding thd->add_status_to_global() before deleting slave thread's thd.


mysys/my_delete.c:
  Added missing newline
sql/mysqld.cc:
  Use add_status_to_global()
sql/slave.cc:
  Added missing add_status_to_global()
sql/sql_class.cc:
  Use add_status_to_global()
sql/sql_class.h:
  Simplify adding local status to global by adding add_status_to_global()

initialize it for full gc if we think it is worth it. This allows for
full gc to run when bringing a basement node up to date.

there may be an abort message waiting to be applied.

update_used_tables for the the where condition to update cached
indicators of constant subexpressions. It should be done before further
possible simplification of the where condition.

This change caused simplification of the executed where conditions 
in many test cases.

in a different order than they began, and blow up a bad assert.

separate the difference between the oldest referenced xid we can use for
implicit promotion vs simple garbage collection.

run full garbage collection when a leafentry has > 1 committed entry.