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(). 

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.

MDEV-4556 Server crashes in SEL_ARG::rb_insert with index_merge+index_merge_sort_union, FORCE INDEX
- merge_same_index_scans() may put the same SEL_ARG tree in multiple result plans.
  make it call incr_refs() on the SEL_ARG trees that it does key_or() on, because 
  key_or(sel_arg_tree_1, sel_arg_tree_2) call may invalidate SEL_ARG trees pointed 
  by sel_arg_tree_1 and sel_arg_tree_2.
    

In function ‘void* memset(void*, int, size_t)’,
    inlined from ‘void Lifo_buffer::set_buffer_space(uchar*, uchar*)’ at sql_lifo_buffer.h:70:5,
    inlined from ‘int DsMrr_impl::dsmrr_init(handler*, RANGE_SEQ_IF*, void*, uint, uint, HANDLER_BUFFER*)’ at multi_range_read.cc:895:62:
/usr/include/i386-linux-gnu/bits/string3.h:82:32: error: call to ‘__warn_memset_zero_len’ declared with attribute warning: memset used with constant zero length parameter; this could be due to transposed parameters [-Werror]

It was intentional that the buffer is set to zero length there.

(gdb) p dbug_print_table_row(table)
  $33 = "SUBQUERY2_t1(col_int_key,col_varchar_nokey)=(7,c)"

- Item_direct_view_ref didn't clear its pointer to item_equal in ::cleanup.
- Some Item_direct_view_ref objects have statement lifetime (i.e. they
  survive across multiple EXECUTE commands). Item_equal objects live only for
  the duration of one EXECUTE. This caused Item_direct_view_ref to have a stale pointer,
  which could cause all sorts of effects. (In this bug's testcase it was pointing to
  the wrong Item_equal, causing wrong query result)
- Fixed by doing what Item_field::cleanup() does - don't keep item_equal pointer value.
- There is no testcase because the only testcase I've got is highly fragile (e.g. the
  bug will not show up if @@datadir is of the wrong length).

Fixed so the MYSQL_TYPE_GEOMETRY is treated as BLOB.

The result is EMPTY for a buffer(line, -1), but we still
need one FALSE operation to be stored in the condition.
And we actually add it but forgot to alloc memory to store it.

- The problem was that JOIN::prepare() tried to set TABLE::maybe_null
  for a table in join. Non-merged semi-join tables 1) are present as 
  join's base tables on second EXECUTE, but 2) do not yet have a TABLE 
  object.
  Worked around the problem by putting mixed_implicit_grouping into JOIN
  object, and then passing it to JTBM tables in setup_jtbm_semi_joins().

(just to have the test in the tree when we merge the upstream fix)

Don't ignore the error from st_select_lex_unit::optimize()

as it was supposed to be in bug#11747970, don't fake an error.
(this kill can be useful for other bugs too)

(mainly by backporting 5.5. changes)

mysql-test/suite/maria/t/distinct.test:
  Remove the test that requires SSL. One test case for a bug is enough.
sql/scheduler.cc:
  make it the same as in 5.5
storage/innodb_plugin/row/row0mysql.c:
  make it the same as in 5.5
storage/innodb_plugin/row/row0sel.c:
  make it the same as in 5.5
storage/xtradb/row/row0mysql.c:
  make it the same as in 5.5
storage/xtradb/row/row0sel.c:
  make it the same as in 5.5

fix the NAME_CONST check to only allow literals, negated literals, and literals with
the explicit collation.

The method Item_field::update_table_bitmaps() should not try
to mark the bit for a self-referencing virtual column.

- Let cleanup_empty_jtbm_semi_joins() walk into semi-join nests.

The field JOIN::select_lex->where should be updated after the call
of remove_eq_conds() in the function make_join_statistics(). This
matters for subselects.

Set of JOIN_TYPE_OUTER made only once to avoid interference with optimization joins which made only once per query.

on server shutdown after SELECT with CONVERT_TZ

It's wrong to return my_empty_string from val_str().
Removing my_empty_string. Using make_empty_result() instead.

Assertion `mon > 0 && mon < 13' failed.'

MDEV-5337: Wrong result in mariadb 5.5.32 with ORDER BY + LIMIT when 
index_condition_pushdown=on
- in test_if_skip_sort_order(), correct the condition under which
  we have the code that restores the previously pushed index condition.

The crash happened because Item_func_monthname was derived from
Item_func_month, so Item_func_monthname::is_null() did not work fine.
Backporting a change from 5.5: Item_func_monthname is now derived from
Item_str_func.

- convert_subq_to_sj() must connect child select's tables into 
  parent select's TABLE_LIST::next_local chain.  
- The problem was that it took child's leaf_tables.head() which
  is different. This could cause certain tables (in this bug's case,
  child select's non-merged semi-join) not to be present in 
  TABLE_LIST::next_local chain.  Which would cause non-merged semi-join
  not to be initialized in setup_tables(), which would lead to 
  NULL pointer dereference.

THD::thd->activate_stmt_arena_if_needed() should be used to temporary activating statement arena instead of direct usage of THD::set_n_backup_active_arena() because possible such scenario:
  1) func1 saves current arena and activates copy1 of statement arena
  2) func2 saves copy1 of statement arena setup by func1 and activates copy2
  3) some changes made for copy 2
  4) func2 stores changed copy2 back to statenet arena and activates copy1
  5) func1 store unchanged copy1 back to statemnt arena (rewrite changed copy 2 so changes become lost) and activates arena which was before.

THD::thd->activate_stmt_arena_if_needed() should be used to temporary activating statement arena instead of direct usage of THD::set_n_backup_active_arena() because possible such scenario:
  1) func1 saves current arena and activates copy1 of statement arena
  2) func2 saves copy1 of statement arena setup by func1 and activates copy2
  3) some changes made for copy 2
  4) func2 stores changed copy2 back to statenet arena and activates copy1
  5) func1 store unchanged copy1 back to statemnt arena (rewrite changed copy 2 so changes become lost) and activates arena which was before.