- Address review feedback: introduce NO_REF_PART symbolic name, better comments

Analysis:
A query with implicit grouping is one with aggregate functions and
no GROUP BY clause. MariaDB inherits from MySQL an SQL extenstion
that allows mixing aggregate functions with non-aggregate fields.
If a query with such mixed select clause produces an empty result
set, the meaning of aggregate functions is well defined - either
NULL (MIN, MAX, etc.), or 0 (count(*)). However the non-aggregated
fields must also have some value, and the only reasonable value in
the case of empty result is NULL.

The cause of the many wrong results was that if a field is declared
as non-nullable (e.g. because it is a PK or NOT NULL), the semantic
analysis and the optimization phases treat this field as non-nullable,
and generate all related query plan elements based on this assumption.

Later during execution, these incorrectly configured/generated query
plan elements result in a wrong result because the selected fields
are not null due to the not-null assumption during optimization.

Solution:
Detect before the context analysys phase that a query uses implicit
grouping with mixed aggregates/non-aggregates, and set all fields
as nullable. The parser already walks the SELECT clause, and
already sets Item::with_sum_func for Items that reference aggreagate
functions. The patch adds a symmetric Item::with_field so that all
Items that reference an Item_field are marked during their
construction at parse time in the same way as with aggregate function
use.

after Monty's review.

Analysis (BUG#719198):
The assert failed because the execution code for
partial matching is designed with the assumption that
NULLs on the left side are detected as early as possible,
and a NULL result is returned before any lookups are
performed at all.

However, in the case of an Item_cache object on the left
side, null was not detected properly, because detection
was done via Item::is_null(), which is not implemented at
all for Item_cache, and resolved to the default Item::is_null()
which always returns FALSE.

Solution:
Imlpement Item::is_null().

******

Analysis (BUG#730604):
The method Item_field::is_null() determines if an item is NULL from its
Item_field::field object. However, for Item_fields that represent internal
temporary tables, Item_field::field represents the field of the original
table that was the source for the temporary table (in this case t1.f3).
Both in the committed test case, and in the original bug report the current
value of t1.f3 is not NULL. This results in an incorrect count of NULLs
for this column. As a consequence, all related Ordered_key buffers are
allocated with incorrect sizes. Depending on the exact query and data,
these incorrect sizes result in various crashes or failed asserts.

Solution:
The correct value of the current field of the internal temp table is
in Item_field::result_field. This value is determined by
Item::is_null_result().

Do not reset the value of the item_equal field in the Item_field object
once it has been set.

Address Monty's review comments

Added item.real_type() for easy access to the underlaying types for Item_ref and Item_cache_wrapper()
This allows us to simplify and speed up some tests and also remove get_cached_item()

sql/item.h:
  Added item.real_type()
  Removed get_cached_item()
sql/opt_range.cc:
  Simplify test
sql/sql_select.cc:
  Simplify test
sql/sql_show.cc:
  Simplify test

Analysis:
The assert failed because the execution code for
partial matching is designed with the assumption that
NULLs on the left side are detected as early as possible,
and a NULL result is returned before any lookups are
performed at all.

However, in the case of an Item_cache object on the left
side, null was not detected properly, because detection
was done via Item::is_null(), which is not implemented at
all for Item_cache, and resolved to the default Item::is_null()
which always returns FALSE.

Solution:
Use the property Item::null_value instead of is_null(), which
is properly updated for Item_cache objects as well.

Don't check if LAST_IO_Error has changed as this is not a user variable and it may change depending on timing issues between master and slave

If join condition is of the form <t2.key>=<t1.no_key> then the server
performs no index look-ups when looking for matching rows of t2 for
the rows from t1 with t1.no_key=NULL. It happens because the function
add_not_null_conds() injects an additional condition of the form 
IS NOT NULL(<t1.no_key>) into the WHERE condition.
However if the join condition was of the form <t.key>=<outer_ref> no
additional null rejecting predicate was generated. This could lead
to extra records in the result set if the value of <outer_ref> happened
to be NULL.
The new code injects null rejecting predicates of the form 
IS NOT NULL(<outer_ref>) and evaluates them before the first row
the subquery is constructed.

- put the code that sets HA_NULL_PART bit back
- Fix test_if_ref/part_of_refkey() so that 
  = NULL-ability of lookup columns does not prevent the equality 
    from being removed (we now have early/late NULLs filtering which 
    will filter out NULL values)
  = equality is not removed if it is ref_or_null access, and the value 
    of the lookup column can alternate between the lookup value and NULL.

Generally, we should use only small letters for table names 
but here it's easier to fix with one --replace.

- cleaner code
- ability to change from using pointers to offsets at some point

The bug was a result of the fix for bug 668644 that turned out to be
not quite correct. A problem appeared with HAVING conditions containing
more than one predicate. If a query with an ORDER BY clause uses
such HAVING condition and the required order can be obtained with
a range/index scan then the HAVING condition has to be pushed into
two different formulas (items). To be able to do it we have to create
a copy of the ANDOR structure of the pushed condition.
 

- The problem was that Mrr_ordered_index_reader's interrupt_read() and resume_read() would 
  save and restore 1) index tuple  2) the rowid (as bytes returned by handler->position()).  Clustered 
  primary key columns were not saved/restored. 
  They are not explicitly present in the index tuple (i.e. table->key_info[secondary_key].key_parts 
  doesn't list them), but they are actually there, in particular 
  table->field[clustered_primary_key_member].part_of_key(secondary_key) == 1. Index condition pushdown
  code [correctly] uses the latter as inidication that pushed index condition can refer to clustered PK
  members. 

  The fix was to make interrupt_read()/resume_read() to save/restore clustered primary key members as well,
  so that we get correct values for them when evaluating pushed index condition.
[3rd attempt: remove the debugging aids, fix comments in testcase]